I just re-read your post and actually, it sounds like she is sleeping through one sleep cycle and then during the REM phase, waking herself up instead of resettling. Everyone goes into sleep cycles anywhere from 20-80 mins long. The beginning and end of that sleep cycle is very light sleep but it sounds like she is waking herself up in that phase of sleep instead of just resettling herself.

Unfortunately, I don't have any solutions but I remember reading about it when DD1 was a baby. Thought it might help to identify the problem though