"What sin has Modi committed?" Singh asked during his address to BJP workers at Sanjay Gandhi stadium here.
The BJP leader is on a two-day visit to bolster the party's preparations for the next General Elections post the separation with JD(U).
Categorically rejecting the JD(U) charge of Modi being an "autocratic" and "divisive" leader with whom political relations was not possible, Singh attacked the erstwhile ally for "suddenly" finding fault with the Gujarat chief minister and his leadership after remaining part of the alliance for 17 long years.
"You have had no problems with Modi between 2002 and 2013, during which his stocks continued to rise ... But all of a sudden you have found fault with him and decided to part ways by citing difficulties in working with him," the BJP national president said.
Modi has been made only the chief of BJP's poll campaign panel chief and not the prime ministerial candidate. "Yet JD(U) chose to walk out by drawing its own conclusion about the affairs of another party."
"Every political party is entitled to make its own decisions , We (BJP) had decided to make Modi our poll campaign panel chief in view of his popularity and charisma to draw votes. What is wrong in that?" Singh asked and assailed JD(U)'s decision to part ways by pre-judging things on its own.