How in the world?

Ella has been getting much better with aiding us in dressing and undressing herself. She can even take off her coat after I unbutton it or get the zipper started. However, I have never seen this!
Around 10:30 this morning she started to act sleepy. So I scooped her up, grabbed her blanket and headed to her room. I laid her down in her crib and covered her up. No complaints. Walked out of the room. Not a peep. 15 minutes later I heard her talking to herself and ignored it. 15 minutes after that she was yelling to get my attention. I walked in and saw her like this...

How in the world did you get just one arm out? Or why?