It appears to me that he’s confident he already has the nomination all but wrapped up, but he knows he needs to attract more votes from the independents and squishies to win the general, so he now wants to show them he’s not a hard right-wing ogre.
The liberals I've known (I've had to deal with them on shared interests) call it “nudging”. It's how they eventually get their way, little by little, piece by piece. Trump will be nudging himself to the left as he feels it becomes necessary. The question is how far will he go before he loses more of his (original) base than he gains with these transitions. I don't think this is going to be pretty. My new tag line, used once, may become obsolete before we know it.