What could cause the battery in a smoke detector to heat up?
Wow- A smoke detector that could cause a fire..

It is very difficult to say


It could be a fault in the fire alarm- short circuit that caused the battery to heat up to a cooking point.
It could be a chemical fault in the battery where atmospheric pressure and chemical imbalances caused by some impurity during production caused the battery to heat up at a certain point of its life.


Either way

The safest option is to replace both and remove all doubt!


   eg ever hear about the exploding iPods...  :)