every so often when i open snapchat on my iPhone 6 plus my phone reboots itself, anybody know why?

Make sure you have no updates pending for this app. Make sure your iOS is up to date. Try a power off, and back on. Do a forced restart:

Close all open apps by double-tapping the home button, then swiping up the app window until it flies off the screen.

Reset your device: hold down the home button along with the sleep/wake button until the screen goes black and you see the Apple, then let go. (No data loss)

If all these fail, check with the app developer for assistance.