Happens frequently to me too.
I have reason to believe it is trying to (un)mount the SD card, can't do it for some reason, so reboots as a last resort.
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Unfortunately, this is a known issue...and as of right now I havent seen anything that addresses a why/how it does this, or a why/how to fix the issue...
My husband's phone was doing that. Tried a few times and it kept happening. Finally got it to work after restarting our computer. Just in case, I saved his pics and backups and reformatted the SD card. Haven't had an issue since.