Your device doing a reset on its own is certainly out of the norm. I do hope you did not lose much. Sounds like you did have the Cloud backing things up which is great. If your device is doing a reset on it's own it is usually caused by an application or software issue. It is hard to pinpoint what exactly may have caused it. Do you recall anything changing on your device before this happened? Any new apps downloaded or software updates come thorugh?
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Mine did this last week.
Out of the blue while I was taking video it showed the Android on it's back with fatal error dialog, then showed me it was deleting, and then did a boot recovery... Basically did a factory reset... That I did not initiate...
I lost some video I took that hour at my daughter's choir concert, but I am OCD on backing up, so loss was minimal...
Just saying this thread is not an isolated one-off corner case.
Having a good working is important, and I'm sorry you're having problems with your device. What's the make/model of your phone? Does the phone have any water/physical damage? How's everything working for you now?