As that behaviour started when you installed all that stuff, it at least suggests a relation. It could be one of the apps causing this; the audio/video files are unlikely to be the culprit.
I'd suggest you factory-reset the device and start over, but do not install all apps at once; instead only install one at a time and see if all stays stable before continuing with the next.
Once the trouble starts over, uninstall the last app you've installed and see if that cures it; if so, skip that app and continue with the next on your list. If all stays stable then, you know which app is problematic: contact its dev for further advice then.
A helpful thing to investigate before going the "hard way" described above is to "debug" the issue. As you didn't state which Android version your device is running, I will describe an approach that should work for most versions:
You need ADB on your computer (see: Is there a minimal installation of ADB? for how to achieve that), and connect your device via USB. Then start
adb logcat -v time -f debug.log from your computer's command shell. Let it run.
As soon as the device reboots, stop the logcat task. Now check
debug.log from bottom to top to see what initiated the reboot. If you have trouble reading/interpreting it, upload the log and add a link to your question, so we can help you with it.