People on twitter are very rude in general. Maybe it's the 140 characters max thing? But I think it's more likely the "I need to vent my daily frustration, and there is nothing like having a go at someone on the internet because I am protected from real retaliation!".
I know it's a very convenient communication method for celebs, but I sincerely hope that most of them don't look and the reply feed from their tweets, so many people are offensive for no reason...