You can backup most of your settings from Settings > Administration > Backup settings so when you do a factory reset you can easily restore and it'll save a lot of time. So you already had V126.96.36.199 installed prior to this?
Yes the lease could have released itself but very odd that would cause it to reset, can you extend the lease please and keep an eye on it and see if it happens again towards the end of the lease time?
I imagine it is probably when the login times out but I'd recommend changing your WiFi details just in case. (I'm assuming no one else but you would be accessing the interface).