I am a mom of two and a preschool director, I feel for what you are going through. It is really hard when your child starts a behavior that is not typical of your child. I have gone through it with my own and seen it with other children.
Preschool is a new experience and learning transitions is learned skill and part of preschool learning. Your daughter is not very unusual at all.
Can you ask her teacher about some of the ways they are preparing for transition? (warning, a signal, song, etc.) I do believe that some children are simply wired to need more pre-warning and signals for transition. I know that we have used yellow "caution signs" as a warning that it is almost time and then a stop sign to signal it is time to clean-up (or whatever), Line up songs that engage the child in activity, picture schedule so children can see what is next, etc. They may already be doing a lot of things and your daughter is simply adjusting and will get the hang of it. If they are, you can talk about the specific signals they are giving in the classroom with your daughter.
Just some thoughts that come to mind right now. I hope it is a little helpful.