If your buttons are being pressed by something that is not the user, you will need to update the project's phsyics settings.
What is likely happening is the buttons are colliding with the enviroment, other characters or even other buttons in the scene.
You can update the settngs so the buttons will only repsond to colliding with the users hands in the Phsyics settings
Edit (Dropdown menu at top of window) -> Player Settigns -> Phsyics
Turn off all collisions for VRUI, except with the User layer.