It may depend on the discharge state of the battery when you start re-charging it, and the battery's characteristics.
If the charger regulates it output voltage but not current, it might be handling a relatively high current for a while if the battery is of a type that demands that at first, when still nearly "flat". A higher current drawn from the charger will warm it more than a low current.
Trying the charger in different mains sockets should not affect it.