Because it's DOING something, and cpu and carrier / wifi chipset heats. Go to settings, BAttery, tap on Show app per use and check
Then, check data sense
<That happens during core apps updates / system sync and cache / Onedrive and backup / Updates pending and stopped due to errors and asking for checking in Store.
And at last, when carrier data connection loses signal stenght. Phone must search another working cell nearby.
At the end of all checking, try a soft reset