Why is this happening to my phone

This is a belated answer so hopefully it can be of use to folks who search for and find this question.

Without knowing the history of this device prior to the failure, it's hard to give a precise diagnostic. I would start by testing a bare-bones setup. Start by disconnecting the battery first (always!). Then disconnect the digitizer, LCD, front camera, rear camera and home button extension flex (see this guide). Now re-connect the battery; you should only have the battery and Lightning Dock flex connected. Connect a known-good, preferably Apple-original Lightning cable to an iTunes enabled computer.

If you have access to a multimeter, you could measure the battery beforehand. If it reads ~3.8V - 4.2V, then it is good. Does iTunes recognize your phone? If so, then you should re-connect one flex at a time until the phone stops working (always power down the phone first). I would start with the Home button flex first, then add the rear camera, LCD & Digitizer then from camera flex.

If the bare-bones setup does the same thing, then you most likely have a logic board issue. You could send it (or find a local) to a repair shop that specializes in micro-soldering repair. Sometimes the fix can be straightforward (i.e. a blown filter) and relatively economical.