It happens all the time if you use more than one device. Try using your phone then switching to a pc. You will need to login again, guaranteed. Can only use one device at a time! you're welcome.
It also happens to me. I think (I haven’t checked) that if you don’t use Private Browsing Mode (Incognito for Chrome users) Imgflip will save its cookies and then you will never randomly log out ever again.