The first version sounds **unfriendly** (*это* emphasizes it) and expresses suspicion of hidden motives. And it's rather a rhetorical question.

> Why on earth would you start taking care of me all of a sudden?

The second one is *more neutral*, but the included **вдруг** also expresses some suspicion or surprise.

> Why (for what reason) did you start taking care of me all of a sudden?