There are several different ways to take screenshots on that model of phone and some are easy to do accidentally. Take a look at this page and see if you might be doing one or more of these without realizing it.
If you have sn6y of your buttons set to trigger the screen shots (or camera) you are likely to trigger it in a pocket or purse. Try switching to either the palm swipe or just using the quick panel button when the you want to take a screen shot.
