Phone launches multi app whenever I launch app

[ZiDanno]

Hi, for some reason my phone launches multi app mode (like when you press the button with the two overlapping squares in the bottom left corner) whenever I click on an app. It doesn't happen every time but when I click on something (say for example what's app) it launches the app but instantly launches multi window where you can close down apps or initiate split screen mode. Is this a bug or does anyone know a fix? I thought it may be some sort of pressure on the bottom left of the screen but it only happens when I launch an app and not when I'm on the phones home screen. Also took the Tech 21 case off to see if that was causing it but it doesn't solve the problem.

Thanks in advance