I'm sure there's not one particular reason, I'd be willing to bet there are hundreds, maybe even thousands of individual reasons that might trigger a restart.
For the scheduled restart in auto optimization to happen, your phone must be idle with the screen off for I believe an hour prior to the restart time. If less than that, the mechanism will assume you are likely to use the phone. If you have used the phone within that time, I believe there is a grace period where the phone will do the scheduled restart within 2 hours of the scheduled time. I know that's been the pattern with my auto-restarts. I have mine set for a time where I know I won't be using it during that time, but if I happen to forget, the only time it does not restart is if I was using it within an hour after the missed time. Do not schedule your restart for during the night, because the night time activity of backing up or updating will be interpreted as "in use" and a restart will interrupt certain aspects of sleep tracking (specifically snore detection) if you track sleep; and may cause any alarm you set not to activate as your phone will be locked after restart and not fully engaged. If you don't track sleep and don't set alarms, and want to schedule your restart for night, check what time your auto backups have been taking place, and schedule for a time about 2 hours after that. For instance, my nightly backups usually take place about 1 a.m. But, they could happen any time between 1 and 3 a.m.
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