tl;dr: try removing custom sample packs via the Config screen in the OPZ app. Especially if the crash is related to a trigger on a track with a custom pack (even if it came from the OPZ app itself).
If you connect your OP-Z to a computer as though you’re using the OP-Z Mac app or routing MIDI to software on your Desktop/Laptop computer, then it makes sense that when the OPZ crashes, it immediately drops into the firmware upgrade mode because that also displays a folder of the crash logs.
I noticed this frequently on my device after playing with the Configure screen in the OPZ app (frustratingly getting different results on different devices on different days). I’ve factory reset my device a few times too, trying to pinpoint some issues.
It seems that my crash logs mention “redmountPerc.aif” so I have a clue as to the culprit. I’m guessing that during transfer from the OPZ app or sometime afterwards (, I can’t tell), this file became corrupted, and the unit just didn’t like it. Ironically, trashing the “redmountPerc.aif” file from the sample packs directory did not remove the pack, and instead I removed it from the Config screen within the OPZ app, and it seems my device is more stable.