I emailed support and they don't believe that the rain sensor caused a reboot because it activated and deactivated again and this time there was no power cycle.
So possibly the electricity went out for a second. There was lightning in the area.
I was not at the house so I don't know for sure.
This brings up another issue / suggestion - it would be great if the Rachio app would provide details as to why a power cycle occurred...for Example AC Power Loss, Firmware Update, etc...