IF:
O-ring is in place on the switch
The switch is not sticking
and you do NOT Over tighten
Then it is likely the Battery went defective.
Could have been dropped at some point and gradually the damage increased to the point of failure.
I doubt it was the Mod, or the Topper. Atomizer short would likely manifest in Excessive heat at the point of the short. Still, best to visually inspect everything and run a meter over things just to be safe.