If the ender dragon is riding a mob or armor stand it will push the entity away with its melee attack. The entity will take a lot of knockback, and because the ender dragon is still on top of it, it will continue being yeeted until it dies or reaches the edge of loaded chunks. This can be reproduced as follows:

1) Open up or create a creative world
2) run the following command in chat:

/summon minecraft:elder_guardian ~ ~ ~ {Passengers:[{id:"minecraft:ender_dragon"}]}

3) Both entities will immediately teleport to the edge of loaded terrain, usually to the south and somewhere up in the sky. (The elder guardian will quickly be killed by the dragon unless it is summoned as Invulnerable or the like, although the changes to mount points in the 1.20.2 snapshots seem to have made this less consistent.)

This probably isn't widely noticed because the ender dragon is (to my knowledge) the only mob that automatically yeets everything in its hitbox, but I imagine it could also apply to mobs from mods which do the same thing. In my opinion the best way to solve this would be to prevent all entities from colliding with their own mounts, as well as preventing the ender dragon from melee attacking its own mount.