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.
This is exactly what happened to me few minutes ago.
I have a thread I update weekly, and I only do the update with a desktop. First, I logged into my account with my mobile phone and when I realized I needed to use the desktop, I switched to desktop. Logged in my account, without checking the always stay logged in button. I proceeded with the updates, after the tedious task, not knowing that I already logged out of the mobile, I wanted to update the thread from my desktop, I was automatically logged out and session expired. Edited post not seen in the draft. I had to started afresh. I wasn't actually happy but I learnt something from the incident.
If you do not check the Always stay logged in check box, you will be logged out of your account if it is logged out in another device.
