This happens because you logged into your account using another device or browser during that time when you closed your first bitcointalk tab.
Logging in to your account using another device doesn't affect your other sessions. I have done that many times and I am sure about that. That's logging out which affects your other sessions. If you log out in one device, you are automatically logged out in all other devices.
Hmm, Strange. It happens every time with me. Note that I always use the stay-logged-in feature. Whenever I log in using my desktop, I notice my account gets logged out from my smartphone browser. I didn't know if that was only me. I guess Jerometash faced this as well.