Bottom panel popped off while sitting down doing nothing.

I left my laptop on and sitting on a table for a few hours and left, when I came back the bottom panel had bulged a bit near the sides. I just popped the back in place again but then I wondered how it got like that. It wasn't dropped or anything, it was just sitting there. I read up on batteries expanding and got concerned, but it has only done 7 cycles, coconutBattery says its in good condition and I have had great battery life, however it is 400 days old. I am confused as to what happened.

EDIT: On second thought could the battery be damaged due to it being stored for over a year at around full capacity? (I think it was around 80% or above when I bought it a few days ago).