There are 2 reasons: The first just got mentioned, overheating by the combined effect of the device heating up itself and other factors like the sun. If the battery heats up too much, the safety system will stop charge, and the battery will then be drained until empty. No soft reset needed, but the device has to be cooled to the appropriate temp and the charger disconnected/reconnected. I was also having that issue when using my older Hermes with 3G sharing over wifi and downloading hard, even with the battery cover removed and the keyboard open the device was heating so much on itself it would cause that. The Kaiser seems to heat less when doing the same BTW.
The second is that when using a common charger (not HTC branded), even if it is rated at 2 amps the phone won't be drawing those 2 amps from it but will limit itself to about 500mA.
The reason is simply that it's being charged from a USB port, and a USB device is not allowed to draw more than 500mA from a USB port. HTC chargers have a chip inside and tell the phone that they are official chargers from which the phone may draw more. Without that identification the phone stays in low current mode.
On the Hermes it was even simpler, if you connected a generic charger it would not charge at all. It would only accept charging from an active USB port it was able to have a communication with, or the original charger.
So, use original HTC chargers when current draw is high. The Kaiser with GPS, full backlight and a GPS program running will draw something pretty close to the 500mA.
Samsung Galaxy S5 G900F 16GB, Stock 4.4.2 (NE2), rooted
Samsung Galaxy S4 I9505 16GB, Stock 4.4.2 (NC4), rooted