I have the Bumbo seat too, and DD pushes back in that. BTW, you can order it directly online, which is what I did. Anyway, we see a physical therapist for an unrelated issue, and she told us that when babies lose their balance while sitting, that is when they arch and push back. It's a process, learning to feel steady and safe sitting up. To us, it doesn't look like they are losing their balance, but their little muscles are just developing and they have to practice in order to feel comfortable. I have a boppy too, and I will sit her on the floor between my legs and curl the boppy around her middle. She will play for a little bit, before pushing back. Then I remove the boppy and try to encourage her to pick toys up from the floor in front of her. I figure when she's really ready to do it, she will, but for now we're just practicing.