Welcome to Android Central! Could be a number of things--conflicting apps, corrupt SD card, glitchy firmware, or failing motherboard.
Try booting into Safe Mode, which temporarily disables all 3rd party apps. From the Verizon support site:
Safe Mode puts your phone in a diagnostic state (returned to default settings) so you can determine if a third-party app is causing your device to freeze/reset / run slow.
- With the device powered off, press and hold the Power button until the HTC logo appears.
- With the HTC Logo on-screen, press and hold the Volume down button.
- Release the Volume down key when Safe Mode appears in the lower left.
This may take up to 30 seconds.
Notes:
- With Safe Mode enabled, test device and app functionality. If the previous issue fails to occur, the cause has been isolated to a third-party app and all recently installed or updated apps should be uninstalled.
- Power cycle the device to disable Safe Mode, reinstall then test each app, one at a time, to identify which is causing the problem.
Sorry, I can't post the link, because Verizon support links tend to be glitchy when copied to this forum.
If that doesn't help, go to
Settings>Storage, unmount the SD card, and remove it, then reboot. See if the reboots keep occurring.