I was under the impression that fuses aren’t actually set until the round leaves the tube.
Not precisely. Yes, there is an inertia trigger to allow the round to get safely away before the firing train is aligned; however, placing another round on top of that fuse could (and apparently did) defeat that safety mechanism by providing inertia and contact simultaneously. It could also have simply caused the fusing to mechanically fail.
Same here. I assumed that even though it was an impact fuse that it had to attain a certain altitude or spin in order to ultimately arm the fuse?
in my 107mm mortar platoon, the rounds had a shotgun shell in the bottom which was set off by a firing pin at the bottom of the tube. if this is the case here the second round would have hit the nose of the first round setting the second round off early