I had a little girl (2yrs) that would do this in spurts. I found that it always happened during times that her own family was going through transition-type situations. She reacted (overly emotionally) in general when this was going on.
I don't take this personally, since most of the time, the kids are TOTALLY fine within a few minutes!
It's completely normal for kids to go through this, its just separation anxiety and them testing their boundaries. I have a little man who cries when mom picks him up. That's much easier to handle, I admit!