my guess...

you were stopped
you, somehow, hit the rear shift lever so that the RD pulleys moved the chain under a larger cog - prolly 2 or more positions over.
chain is still engaged on the small cog as you get out of the saddle and push down.
however chain now also engages the larger cog from underneath, while still remaining engaged on the small one
these chain plates, by their design are less twist resistant than solid plates.
chain is now twisted and as it progresses forward, the first twisted link rides onto the top of the ring tooth it meets and bucks upwards causing the chain to catch/bunch on the FD roof...

there is a chance that a similar scenario could potentially happen if you had hit the front shifter and the FD had moved the chain over so that the chain became engaged on both the outer and inner rings at the same time, again causing the chain to twist. Given the twist, the inner ring because of pressure of the twisted plate onto the tooth, might not have allowed the chain to release under pressure - hence causing the chain to bunch up...

in either case, I would expect that there's a real possibility that the FD cage alignment might be mucked up... I'd check for that, for sure.
don't think there would be any issues with the rear jockey assembly...