Champ Select

Was spectating a friend's game and when I went to exit LoL just disappeared. Didn't bother to check processes, just opened a new instance and let the interface close the other process. Joined queue with my friend and shortly after champ select came up I received a message telling me my account was logged into from another computer. LoL closed itself out so I started it again and rejoined queue, 15 min ban. This was the second time I have ever faulted out of a queue, the first was about three months ago. Why is this happening and why is it 15 minutes for something on your side. THANKS