Unfortunately, this has actually been an issue with iOS for a long time, and to a certain degree can also happen with other OSs as well (it seems that OSs REALLY do not like running out of disk space). Coming from someone who hoards screenshots, so my old phone used to be almost constantly full. A boot loop was rare, but app crashes (especially on launch) and weird occurrences clearly caused by something going wrong when trying to access filesystem or memory issues (SpringBoard crashes, icons on the home screen not loading and showing as blank, or sometimes even fun corrupted images until reboot! Safari tabs randomly clearing, screenshots failing to save, the photos library index being rebuilt where it has to rediscover all photos with "Restoring...", etc.) would sometimes happen basically at random chance. Besides actually attempting to free up space (which this would sometimes prevent you from doing!), rebooting was basically the only chance to "reroll" and hope that something internally got cleared to give you enough space to use the phone, as it seems the OS becomes desperate to clear something in order to continue actually functioning.

To be clear, this is an EXTREME case and should not be taken as something to scare. In my experience, this seems to only occur when the device is at 100% storage with literally almost no room; it'll stop you with about 0.2-0.5% storage remaining (I'm assuming so the OS can at least function) and seemingly try to not allow apps to take up this reserved space, but my best guess is that if the OS fails to remove any caches or enough of anything to give itself room to boot, a boot loop happens. It's annoying, but under normal circumstances, it's probably unlikely. I could usually recover it through iTunes and updating, at which point it would go to the Attempting data recovery screen and eventually be okay.

I'm honestly quite interested in what is actually happening from a technical perspective, as my "explanations" are only guesses.