Well that has a whole lotta questions and continuity issues. But I guess the big one is like with TF4 having the new Decepticons turn into millions of little cubes to transform - having Bumblebee pull himself back together is done for the visual moment of the scene, even if it contradicts previous movies... like how that has killed every other Bot who's been blown to pieces. Sure, it looks good and amusing to have him pull himself together, but rant rant rant rant....

Okay, I'll wait until the movie comes out, and hope a plausible explanation is given.