Interesting, I was the first person that I saw experience this but you're now the third person I've seen mention this happening. No idea what causes it. The only way I could fix it was to restore the phone.
I also was having the same issue, unresponsive, crashing, stuttering, hitching, rebooting, etc. sometimes would have it sitting with nothing running on charge and i would hear the charge connect sound from it rebooting. 13 Pro Max, nothing specific triggered the issue, just happened everywhere doing anything and seemed to get worse as time went on, plenty free storage too (1TB model with like 150GB used).
Tried reinstalling using the 18 beta iPSW, made no difference, was planning to just downgrade to 17.5.
A couple hours ago I accidentally dropped my metal water bottle on the phone from a fair height, completely smashed the front and rear. (yes I had a screen protector and case on, I also have AppleCare+)
Have had pretty bad luck recently =(
Will be sticking with 17.5 for a little bit post repair.