Hi Mark,
When you hover over things that can be left-clicked with your mouse, is it automatically clicking them? If not, it definitely doesn't sound like anything to do with your mouse buttons being held down.
Beyond that, the suspicion goes to the gamepads and the keyboard. You could go into the keybindings and remove the gamepad/axis bindings for the regular ability 1 if you like. Or if you've not restarted the game since unplugging the gamepads, you may wish to do that to fix it; unity only notices gamepad removal/additions when it starts up.
Actually having the analog stick set to the right or left improperly on your gamepad (such as it laying upside down and hitting the analog stick when it was plugged in or your computer powered on), would cause this. The xbox 360 controller, which this game is defaulting to these days, uses ability 1 via its left trigger. That same mapping in unity corresponds to left movement in most other gamepads or joysticks.
If you don't plan on using a gamepad or joystick, you can just unbind those in your settings file and not worry about it any more at all, of course.
Hope that helps!
Chris
