Nutloose,
The annexing is all a bit heavy handed, since there's been no threats from the new Ukrainian government.
However, those Russians have lived in Crimea for many generations. It's their home, and there's no reason why they should move. They can't have dual citizenship now they're Ukrainians, but since they have a clear Russian identity, and there's the possibility of instability in the future, it's not unreasonable for Moscow to take an interest, or to suggest that the region might be better reassigned to them, if that's what the people want.
There's also the fully legal, legacy Russian military bases in Ukraine to add further interest in the area to Moscow.