As the title says, my lipo puffed up while storing.

Lipo is 5000 mah 25c. Never have I ever drained it down below 2500 mah. Pack is fairly new with about 15 cycles. Yes, it's always balance charged and yes it's not stored fully charged.....mainly at 7.8v. No I don't store it in the warm or cold place, just at room temperature. Run the pack on my 2wd buggy. Speedpassion ESC and 7.5 motor. Normal use...never abused, ran my car indoor. Took the battery out last week to prepare for my race meeting and noticed the battery swelled up badly. Yes, my charger is a good quality one.

What could have happened