Cue the Resident Trump-can-do-no-wrongers to try and convince us that there's some master chess move at work on this one...
I'm one who
thought Trump was a terrible choice for president.
Seeing the results he can get, I no longer just dismiss actions like this as him being thin skinned and petty.
It might very well be the case, or it might be that he's trying to accomplish something else (create drama/entertainment?)
I'm not a "Trump-can-do-no-wrong" but I'm one who has to take a step back and wonder if it isn't primarily an act. The fact that he doesn't just attack everyone who maligns him suggests he has a reason for it. It might be "He's just wildly inconsistent and attacks whoever gets his attention" or it may be that he chooses his targets purposely (or instinctively). (Has he ever attacked the Parkland kids? I can't recall, but if he has, it was only after holding off for a long time. Again, that's not fitting with "he attacks whoever attacks him indiscriminately.)
Given that he, more often then not, ends up looking better than whomever he engages with suggests some kind of selection process. I.e. there may be a method to his madness.