Something broke in the last patch with getting knocked off your horse.
If another lancer on a warhorse hits you with a blunted or decorated lance you fall off your horse and get stunned for 5-10 seconds unable to do anything even with Iron Grip activated.
This means you can just go full speed into another lancer and whoever the server decides hit them first auto wins the fight.
Here's an example of the stun:
https://youtu.be/ZXZ3sYlHP4k
https://youtu.be/YXdWs6ZMFfs
Example of the bumper cars
https://www.youtube.com/watch?v=G-NbMOiC1QM&t=91s
I'm 90 cav 90 knight 90 lancer with full t2 chain on a heavy warhorse and had it happen 100% of the time when fighting certain players. They hit you in the leg or run into your horse and you go flying then stunned.
It's pretty broken and stupid. Perma stunlock shouldn't be in any game.