I think it's an instinctive reaction. My big female RES used to do that all the time whenever i picked her up. Now she doesn't seem to mind anymore and she just looks at me like, "where are we going now?" But if I put my finger in front of her she gets the insane "I'm starving and I'm going to eat that!" look and tries to get a good bite. It's pretty scary, really.

Aside from that she's always really nice.

The two smaller males are a little shier but they don't try to bite and they seem to be more comfortable around me now.
I think it's a phase that they go through. Just continue to be calm and gentle with them and if they seem irritated more than usual, keep the handling down to a minimum for a while.