Doesn't it also align with him becoming the presumptive nominee? I would think that's more the reason than name-calling.
I was thinking that as well, but the indiana primary was May 3 and it was flat for a hile after that. Also, the drop for Hillary is much more pronounced than the rise for Trump.
There are other possible factors as well including the tension with Bernie. The question is which is the cause and which is the effect.