Well I know my HH4A reboots nearly every 14 days, I think it might checking how long its taking to do stuff internally and if its becoming a bit slower than usual it does an IPC Reboot, which is why it can and has lasted 19 days a few times.
Or it might detect an issue and schedules a IPC Reboot, I know the Home Hubs are always receiving settings and maybe one of those tell it to do the reboot.
The annoying thing is it doesn't check if anyone is using the connection before doing the reboot, I once had it reboot on me when I was downloading 36GB and it seems it got about 34GB when it did the reboot, sadly the server didn't support resume
And it takes forever to download that over an ADSLx connection with a few Mbps.
So whenever I have to download a huge file(s) I wait till after the reboot.
Yeah I was in a queue for 37 mins on last Saturday.