I am not 100% positive why Amelia does this when she sees (or holds) dirty diapers...but she does.
I think it all started when I had a dirty diaper, in a bag, at my mom's and I was spinning it through the air while I stood in the living room and talked. I looked over and Amelia was also spinning. I thought it was a coincidence but I am not sure because now whenever she sees a dirty diaper she spins. Sometimes (like in the video) she will pick up the dirty diaper and spin with it. She only does it at my mom's. It's all so strange but pretty funny too!