I don't have much experience being diapered and trying to sleep but if I had to say why the little side comes out I think its thanks to the fact you cant just fall asleep right away.
Like you body is not used to sleeping at that time so your just stuck there thinking (which in the right frame of mind will trigger more little thoughts), which is why the little side comes out since most kids are not 100% sleepy when they are told its nap time, so when they are told to go to bed they cant just go to sleep right away since there body is still awake which then leads to a point of just thinking before falling a sleep.
Side note, it has been said that if you try to force your self to sleep by thinking it you tend to not fall asleep right away but it has been said if you try to keep your mind open and think about anything/ everything you will fall asleep much faster.