Toddler refusing to lay down for sleep for months now

It’s been months and my toddler who was an independent sleeper decided to one day sit up instead of laying down to fall asleep and never looked back. It’s been months. 4 to be exact. He refuses to lay down on his pillow, like he always does, and falls asleep. Instead he sits up against his headboard and falls asleep by bopping his head back n forth nonstop for f*cking hours before falling over only to do it all over again.

We have such positive sleep habits that we go through every night. I don’t know what I’m doing wrong or can find any info as to why toddlers do this. Has anyone experienced this with their toddler? Looking for a friend here.