It was an extra safety feature added with iOS 15. Basically, your iPhone monitors battery temperature, charging coil temperature, and ambient temperature inside the iPhone. When one or more of them reach a certain temperature when charging, iOS pauses charging cycle to allow everything to cool down.

If your iPhone is busy doing something like restoring from backup or installing an iOS update (for example), your iPhone would get quite warm and that could cause the overheating while charging message.

Using non-certified chargers could also cause overheating as they are notorious for overcharging iOS and Android devices.