Which model hub?
I ask because about a month ago, smartthings updated the firmware in the V3 hub to turn on the thread radio. Which is actually the same radio as the Zigbee radio, but the two are now taking turns. This caused a bunch of issues for some community members with their Zigbee devices, including some random turning on and off. They tried to fix it with a new hub firmware update, which didn’t fix it right away, although I think it’s supposed to be fixed now after yet another update.
Anyway, the point of all that is that quite a few people were having problems with Zigbee Devices in November, and December, But of course that doesn’t explain the other issues unless there was a triggering issue with the Zigbee device. 
There were a number of different threads about the issue, the following was typical:
the issue was also mentioned in the official release notes for the hub Firmware updates.
Aeotec Smart Home Hub/2018/2015 Model Hub Firmware Hotfix Notes - 0.45.11
Again, I don’t know if that has anything to do with the problems you were seeing, but I did just want to mention it as the timeline is similar.