If you are blocking or attacking you will be dismounted - that's intended. Now, I play on mouse/keyboard, so I have block and light attack maps to pressing my mouse keys and I turn the camera by turning the mouse. You might try checking your controls and keybindings to make sure you have your controls the way you want them because if you have the same key bound for light attack and camera control, you will be dismounting yourself. If, however, the keybindings ARE how you want them, I'd maybe check your mouse drivers if you think the mouse is sending input it shouldn't.

Regardless, I'm on PC/NA using a mouse and keyboard. I dismount when I block and light attack, however, I can ride around and turn my camera with the mouse without being dismounted because I'm not using those keys.