There were stories about Clinton's signing the agreement w/the DNC back in 2015; I read about it on Politico. There was nothing illegal about it. She signed it in August 2015 — and Sanders signed the same agreement in Oct 2015. 

It's a moot point anyway. If Sanders had got the votes, the DNC would've switched support to him. That's what happened in 2008. Clinton was the front runner, but the Obama started gaining, winning primaries, and the Super Delegates switched their votes to him.