My LB does the same so thanks for asking this question! it looks so uncomfortable when he does it.
It's probably because they're just more aware of the world and want to be a bit more upright and see what's going on. They shouldn't *need* a new seat if they still fit in the current one. But they may find it more comfy. We're going to get the next stage seat now as car journeys have become a nightmare with lots of screaming and crying until he's sick...hoping the new position is the answer. We're going for the axkid minikid 4 so he can be fairly upright
My little one does this, I think he just wants to sit up😅 I remember my first going through this phase too, it’ll pass!