Why does my phone randomly vibrate & start exiting me out of apps?

I could be typing a message and my phone will randomly start exiting out of apps as if I were constantly pressing the back button. Sometimes, it will even send a message when I'm not done with it. And, on the home screen, it will take me directly to the first page and I can't seem to click anywhere on the screen or it will just vibrate continuously. To stop this from occurring, I have to press the power button to put in sleep mode & then unlock it and go back to what I was doing, but majority of times it just continues. Why does this occur?