I don't have a problem with the rexes following junior back to the trailers (they even say in the film that Tyrannosaurus had an extraordinary sense of smell), what doesn't make any sense is when both of them track Sarah miles across the island based on the smell of their infant, when they already knew exactly where it was. Did they suddenly become so stupid that they couldn't count how many babies they had? Or are these dinosaurs just hell-bent on revenge, making them the movie monsters that Spielberg was so keen on avoiding in the first film?
Anyway, as for Sarah's motivation, that's one of the very few sequences in the film that's lifted from the novel. In said book, Sarah isn't a palaeontologist at all, and the only member of the party who is doesn't find out about the abducted youngster until it's too late. In her defence though, it's pretty rare in nature for animal parents to track down their babies when they're driven away in a car.
:edit: Holy flying spaghetti monster, 8 ninja posts.
