I've heard several reports about stepper drivers getting damaged by disconnecting the wire to the stepper motor while it's running, but I never fully understood why this would lead to damage.
no fancy explain here, every inductance that is disconnected from supply fights the current interruption and generates a spike. if driver is mosfet-based, overvoltage means kicking the bucket
So I'm missing something, but what?
from my experience, mosfets die very quick if voltage specs are not respected (overvoltage i mean) or second cause, overpower. a little spike is deadly for them, unlike the bjt that can handle better spikes. you can try with a couple of mosfets yourself.
think of this, a loose contact for example (which is not the primary cause of death in industry, there you got pro connectors or even soldering, i imposed myself soldering in a company using mosfet drivers for motors, never had problems, the primary cause there is DC rail fluctuations or mechanical problems of controlled motors) will tend to generate a spark like you say, but the spark has enough time to discharge some energy on transistor, it's not like a clean cut when all the energy goes into spark, that's the gap between theory and practice. the motor bobbin will definitively produce overvoltage in driver, no connector disconnects like a professional designed contactor/disjunctor.
i think you are tackling this too theoretical. i spoke with other engineers with some experience in power electronics, they confirmed me this overvoltage explain (more like a field engineer myself, but also some r&d in that motor driver business, mostly for dc/step motors/brushless, the big part was step motors driver design, but not for huge motors)
for me this was the only logical path because the drivers i found burnt had no mechanical blocks in general, no mechanical overcharge, and the scenario was mains voltage fluctuation (so you have large dc rail variations) and sudden mechanical block of drived assembly (this push us to overvoltage, the scope confirms this). for the OP case, loose contacts means the same thing (well, that's for the hobby case)