It started because of the frequent crashes the Xbox gets. If I play for ~1.5 hours, it will eventually dump me out to the Xbox dashboard. So I get in the habit of saving and quitting then relaunching to keep gameplay smooth. Occasionally one of those crashes will be an MD5 crash...which requires I delete my local save data and relaunch. So to prevent that, I so the quit and reboot frequently.
I discovered the horde reboot when I got a crash in the middle of horde night. When I relaunched, it was like a completely new horde...different directions...different patterns, etc.
Then I tried the relaunch when I noticed the horde slow down (2am-ish), and same result...new horde, new directions, and lots more Ferals and Cops.