I suspect the water was getting low on oxygen and the fish would do anything to try to find better water.
I came upon a similar situation one winter while deer hunting a late season. The backwater ponds were separated by an old gravel roadway. there was a bit of open water current flowing across the gravel, but not enough for a fish to swim in. The crappies were leaving the one pond (dead end shallow pond freezing almost to the bottom) and attempting to reach the other pond ( pond with current and connected to a creek).
There were crappies everywhere flopping on the ice and exposed gravel. Many had died but some were still flopping. I’m sure the raccoons had a feast that night.