Frequently, her attempts to wake up get foiled, and she falls asleep before putting her arms back where they belong. They get marooned up near her head, covering her ears, and she sinks back into peaceful slumber.
I think it's some sort of ancestral memory encoded into the mitochondrial DNA; I blame her Nana.* After all, her hair isn't long enough to cover her ears, and perhaps in sleep she is more susceptible to strange whispers from the past.
Or she's trying to tear her hair out, no doubt muttering to herself in baby talk "how did I come to be born into such a family of fools?"*
*It seems likely that she would think that my (our?) obsession with ears is foolish, but that is no doubt because she has not lived for years with honking great ears that stick out from her head. And, unless she decides to have plastic surgery to impose that defect, never will, and so will never understand the problem of enormous, protuberant ears. And she should be grateful.