Mitt Romney follows Harry Reid's lead on questioning Trump's tax returns
I suspect Reid and the Democrats wanted to wait until Trump was the nominee to spring this demand on him since they view him as easily the most beatable of the candidates in the GOP field. Reid famously made up stories about Romney's tax returns. His bogus claims were eventually discredited, but I am sure he is not above doing that again, and Romney's moves preempts the attack and also forces Trump's hand.
Mitt Romney emerged from political hibernation on Wednesday and issued a challenge Donald J. Trump: Show us your taxes.