Villagers having a passenger randomly go full speed and run around. It seems like they are panicking.
It can be reproduced by simply mounting any entity on a villager:
/ride @n[type=minecraft:chicken] mount @n[type=minecraft:villager]/ride @n[type=minecraft:chicken] mount @n[type=minecraft:villager]Environment
Fresh Vanilla 1.21.5 world
Can confirm in version 1.21.6