Check keybinds, get a key for Dismount, unmount, or leave vehicle, something like that. If that doesn't work, make macros for everything you use, containing /dismount, get an addon that blocks texts on your chat by word, and write the message spam in the addon so that you don't see it everytime you press the macro, or it will say "you are not mounted". If you are super low level, start a new char, if you are normal level, check interface options and check the autoDismount on Flight option. This doesn't happen to people.