Now im not an expert in this matter because I’ve never experienced this In my riding. It could be from idling as riding backwards can loosen your pedals, although you would think the idling motion would loosen and then tighten. If you have another set of pedals I would try threading a new one on to make sure you have not damaged your crank ( steal one off of a bike if you don’t have an extra just to test the crank threads). I have made it a point to quickly go over my unis prior to each ride, I have found loose seat / handle bar bolts as well as pedals and just last week a loose crank bolt. It only takes a few minutes with my 3 way alen key and my udc pedal wrench . Hope it’s not your crank that’s stripped. Good luck.
If you ride with it while it’s loose you’ll damage even the best quality product. Aluminum is softer than steel, so after the pedal loosened up it was what suffered the damage. ISIS cranks are a stout design however. Just order some new ones and chaulk it up to a mistake.
And backwards riding and idling WILL tend to result in loose pedals. What I’d advise is to kind of shake, bend and squeeze the components of your unicycle before each ride. I’ve had pedals, cranks, bearing housings, hydraulic brake fittings and seatbolts loosen up. Fix the problem right away and you won’t have any damage.
Looks like you’ve been riding for a while with pedal loose to cause that damage, surprised you didn’t feel it clunking around through your foot? Recommend regular check of pedal tightness once new crank fitted ( I loctite my backwards unicycling pedals)
If your not very mechanical minded make sure you use correct crank puller otherwise you’ll have issues with tool stuck in the main hub spindle, many threads on here were people have done so.
You might also want to check the pedal’s thread. While you’d rather expect the crank’s thread to be damaged due to it being the weaker metal, it’s also possible that the pedal’s thread took damage. A pedal with damaged thread is more likely to damage your new or repaired cranks. So if you can see any damage on the thread or it feels particularly loose while screwing it in a good crank, you might want to replace that pedal.