When the ender dragon is riding the wither, it causes them to move in one direction very fast.

Steps to recreate:

1. Run /summon wither
2. Run /summon ender_dragon
3. Optionally run /ride @s mount @e[type=ender_dragon,limit=1]
4. Run /ride @e[type=ender_dragon,limit=1] mount @e[type=wither,limit=1]

Extra information:

When I run the command blocks like in the recording, the wither and ender dragon fly south. However, I did this without command blocks originally (manually running each command) and the ender dragon and wither went in a different direction. I suspect it has to do with which way the wither is facing when the dragon starts riding it.