Are you using gestures? Are you maybe accidentally swiping in the area that would open the recent apps menu? Are you using the nav bar? Are you accidentally brushing the recent apps icon? Or are these things happening when you are just sitting there looking at your phone and reading a web page or something? We don't know how your phone is set up, or what other apps are on it to be able to answer the why. At the very least, try rebooting your phone and/or deleting the cache partition and seeing if this behavior continues.
