Battery most likely. If the voltage drop before was too low for too long it happens. I’ve seen it frequently. They can still expand/vent many days after the initial event so discard of it asap.
Had it happen to one of mine and put it down to a different charger although the charger supplier figured it was the battery had reached the end of its natural life.
I’m sticking with the trusty CTec and we’ll see how this one goes.