For many, it was surprising for the same Banerjee had asked her own party members in 2013 not to talk about the AAP, which was perceived to be a threat to West Bengal's ruling party in the Lok Sabha election that followed in April-May 2014.
But is Banerjee's support for the AAP now really a surprise? Even if the common enemy called the BJP wasn't there, the Trinamool supremo's backing the AAP is not really an unexpected turn of events.
Trinamool & AAP: Not much different
There is a certain similarity between the TMC and AAP, even if their areas of operation are different.
Both parties rode a massive anti-incumbency wave to power
To begin with, the evolution of both these parties and their top leaders have been phenomenal for they rose as the ultimate symbols of hope in their respective territories after the establishment showed signs of absolute decay.
In May 2011, Banerjee saw the end of a 34-year-old regime of the Left in Bengal and earned accolades from all over the world. In 2013, Kejriwal's AAP also made a historic political debut in Delhi as it won 28 seats in the 70-member assembly. Kejriwal himself defeated seasoned politician and incumbent chief minister Sheila Dikshit, who served for 15 years at the helm.
While state oppression was Banerjee's political capital during her long struggle, Kejriwal made anti-corruption movement the launching pad for his rise.
Both Mamata and Kejriwal struggled to meet the huge expectation
But, the tremendous expectation that these two leaders had once generated soon started to burden their soldiers and being first timers in the administration, both Banerjee and Kejriwal showed signs of flawed capacity. Another striking similarity between these leaders is that they refused to shed the rebel in them even after coming to power.
Both TMC and AAP supremos believe in personalised style of Naxalism
While Banerjee took on the society at a large over issues like human rights violation and deterioration of law and order, Kejriwal even went a step ahead and took on both the state and the society.
Both refused to give up their 'common man' tag even while serving as their leader and it caused a massive inconvenience to the ideas of good governance and executive operation.
When Anna backed her as PM and criticised Kejriwal, why didn't Mamata differ?
Both these leaders also believe in a personalised style of Naxalism which is a complete anti-thesis to the idea of governance. Refusing to acknowledge problems or creating an issue of them instead of resolving them have been a common feature of functioning of both Banerjee and Kejriwal and ironically, the common people have benefitted little from that.
Both leaders attack State institutions at will
Both leaders are known for attacking institutions of the State, whether it's a post or an occasion and in embracing such unsettled styles of politics, they have empowered and encouraged those who disregard law and decorum.
We have heard Banerjee saying openly whether she would beat up the prime minister (then Manmohan Singh was the incumbent) if the latter did not listen to her demands for the state while Kejriwal was seen announcing openly that he is an anarchist.
Besides similarities in their supremos' personalities, the parties also exhibit some similarities and how they have fared in the acid test of credibility.
Neither TMC nor AAP have any ideological bonding, both are assemblies of individuals
Both the TMC and AAP are congregations of individuals who have little ideological similarities. These minds came together only to fight a long-persistent threat (Left's oppression in case of TMC and corrupt UPA government in case of AAP) but in both cases it was seen that these parties began to experience an internal rift as soon as the external challenge had vanished.
Both parties have seen serious dissent over the past months
Both the TMC and AAP have seen a thinning membership over the past 8-10 months.
While Kejriwal's sudden resignation from the chief minister's post created a deep sense of dissatisfaction in the ranks, the TMC's steep fall after the Saradha chit fund scam has seriously dented Banerjee's biggest political asset, i.e., her image of honesty.
But this was not surprising for neither of the two parties ever had any ideological glue to keep them together. The result could be seen: The dissent and the exit.
Banerjee's U-turn: 2016 election in mind?
Banerjee has taken a U-turn from her 2013 stand because her party is in a bigger jeopardy. Thursday was a bad day for the TMC supremo in office for not only two of her party members, including a Rajya Sabha MP resigned, she also received a blow in the Supreme Court as it refused to interfere in the Saradha scam.
In this situation, an increasingly cornered Banerjee is perhaps most concerned with the 2016 assembly election.
If AAP reciprocates in Bengal in 2016... Mamata hopes this could be the way to catch the middle-class votes
The growing distance with organiser Mukul Roy might have left her deeply worried over the preparations and mobilisation ahead of the next assembly election and backing the AAP could be a blind effort to get the blessings of the middle-class in Bengal which has become increasingly disillusioned with the Trinamool government, thanks to its pathetic performance on all fronts.
More than feeling happy to see the BJP defeated in Delhi, Mamata Banerjee will be glad if Kejriwal decides to back her in the next assembly election in Bengal as a sign of reciprocation.
The fight between the AAP and BJP is certainly not going to end in Delhi and particularly if Kejriwal wins, the prestige battle will only snowball in other parts.
Mamata sees a bigger opportunity in the prestige battle between Modi and Kejriwal
A beleaguered Banerjee sees an opportunity in that battle to remain relevant in her own den and she is backing Kejriwal who is still seen as a clean leader by a considerable section of the electorate so that her own moral high ground does not get submerged by the growing waves of anti-incumbency.
Indeed a sorry state of affairs for a mass leader like Banerjee. But her support for Kejriwal is not out of the blue.