Pretty common. If it is an IRO build it likely has a SOMA cog. Your cog got snugged down tighter from pedaling than it is possible to do with a chain whip. Then as a result the lockring is loose. When you brake the cog slips until it meets the lockring. When you pedal hard it tightens again. This happened to me on my IRO and I knew what it was. I pedaled the cog on as tight as I could and then retightened the lockring with a lockring spanner. It has been fine since. Blue loctite on the lockring also helps. But if you get both the cog tight as possible and then the lockring it should be fine.