... Wheeljack dropped his sports car mode and went back to the hovervan because the car didn't have enough trunk space for all his junk he hauled around.
Tangentially, Wheejack's Cybertronian altmode never made sense to me in the context of everyone else's matching their Earth mode pretty well. ("Everyone else" being like four guys, admittedly.)
Storywise, I guess Wheeljack makes more sense in that scene than Ironhide or Ratchet or whoever, but it's always bugged me.
Toy-wise, it's interesting that they gave the engineer-inventor role to a sportscar, which is where this rabbit-hole starts.