goes back to home screen

i have had the iphone 6S for about a year and a half and it cracked in july of 2017. the home button later fell out in around october 2017. now i am using the "control center" home button. my phone started going back to the home screen while i'm in apps or switching to different apps randomly almost like i'm doing it myself, even though i'm not. why is this happening? could it be water damage? how do i fix it?