This was on my phone. My brother and I both play this game. I was kicked out (I was signed in when I went to bed and when I woke up I was signed out) of the session. I was about to hit log in when I realized I was about to sign into my brother's account, MlemMlem. Either he's used my phone before (without my permission the jerk), or something else happened.

Not sure what kind of help we can provide here. We can't control the transfer of login data between your devices. You either use some type of software that synchronizes browser data between different devices (for example, Google Chrome and Mozilla Firefox can do this if you tell them), or there's a human factor which you already mentioned.