Windows 8 now has touch gestures which are available on multi touch trackpads. If you slide your finger in from the right of the trackpad the charms menu will likely appear. It happens to me a lot as well. Another thing that happens with me is the start menu popping up. This is because if you open the charms menu via a gesture and then let go, the start menu will open as it is selected by default. Give it a try and see if it is what you are experiencing. If so you might just want to disable that gesture.
Have a look at your mouse settings in Control Panel. There will likely be a setting which modifies how the trackpad can be used. It is different for each driver so I can't give you very specific instructions.
This may be a solution for you: how do i turn off windows 8 gestures or on Superuser (a related question)