The Bhagat Singh coins were due in 2007, during the release ceremony sept 2008, there were objections that Bhagat Singh [a Sikh] was depicted using a legendary Hat rather than traditional turban ... it went on for quite sometime, later the Proof & UNC sets had major printing errors in packaging [Shaheed Sagat Singh] ...
In 2010 an MP raised a question in parliament as to why these coins are not released for general circulation, the mint said that right now they are having their hands full [10 issues in 2 years] ... subsequently the Hyderabad mint put up for sale quite a few single coin sets to encash on the popularity of collectors ... in this run, they also decided to run the Bhagat Singh coins for general circulation as well.
So the actual run for circulation happened in March 2012 and large number of coins put into circulation.