When a thrasher attempts to grab a player in multiplayer, it effectively bans a user from the server. No client side errors are produced, but you are disconnected for an internal server error each time you reconnect.
Something about how the thrasher handles forcing players to mount it creates what is presumably a de-sync or corruption to player data on servers, which causes the client to disconnect.
I have not been able to recreate this in singleplayer mode, but it has happened to two separate players on the same server.

You're gonna need to provide a lot more info like versions, and logs for both the client and the server