The answer to 'why is my junction box hot to touch' is that a hot junction box is a sign of an electrical problem that requires immediate attention. This could be caused by loose wiring connections, overloaded circuits, or a short circuit in the box. The heat indicates the flow of too much electrical current, which can be a serious fire hazard.
If your junction box is hot to the touch, you should turn off power to the box immediately and call a licensed electrician to diagnose and fix the issue. Attempting to inspect or work on a hot junction box yourself can be extremely dangerous and should only be done by a professional.