How can minibot battery short circuit when it isn't on or being charged?

[Techhexium]

I'm not in FRC, but when my FTC team meet today to build a robot (not a minibot), and one of our batteries short circuited without being on or charged so one side of the battery was burned. The 20A fuse did not blow, the battery was plugged into the motor controller through the connector wire but not the power switch so we can't turn it on, and the robot was not in motion. The battery is a year and a half old, and we have been charging it in 1.8 Amps. (I know we should be charging it at 0.9 Amps) Also the battery was being secured by a couple standoffs on the right and a channel on the left. Sorry I don't have pictures.

My coach told me that the battery would have burned like that if it was charging, but in fact it was not being charged or used. My question is how was it possible for a minibot battery to short-circuit if it is only connected to the controller through the connector but not to the power switch so it can't be turned on?