This happened during F&W for the last set of the Dennis DeYoung concert on 10/29 [around 8:20]. It was quick but still notable that something misfired. Dennis actually turned around to his wife [who is a backup singer] and you could see him say, "what was that".
Unexploded shells. It happens. They go to reset for the next show and it is safer to set them off in place then to remove and dispose. The shells contain a fair amount of explosives.