Is it the Moro reflex or just jolting herself awake, like when you feel you are falling as you're dropping off?
Mine is 19.5 months and even now, when she is working on a new skill, she starts jumping in her sleep and waking up. She has always twitched a bit while falling asleep, but this also gets worse when she is working hard on learning something during the day.
Learning to walk was the worst time for sleep disruption - she was all over the place!
I think she is just a very active, bright kid and can't switch off from the intensity of daily development sometimes. I wondered if since your LO is entering a stage of rapid physical and mental development, it could be something similar?