I imagine he just wasn't thinking. Horrible penalty for a moment's lack of attention . . . but, you know, we've all had moments like this in reloading or in shooting oddball stuff. It's just that circumstances didn't combine to produce such a dreadful result. My husband and I reload together and check each other's work . . . we have occasionally found a round that weighed short or a bullet seated too deep, despite all our care. I of course have had plenty of case ruptures with the Lee Enfields, but since I use moderate loads and the receiver is very sturdy, no harm done. And since I started neck sizing only (and consequently assigning 50 pieces of brass to each of my rifles for use in that chamber only) I haven't had any more problems.
I have stopped using surplus ammunition from WWII, it's just getting too old. Even when you break it out of an apparently untouched case, it's got corrosion on the brass and just looks crummy. The Greeks continued to use the Lee Enfield up into the Cyprus conflict, and I've run across some Greek stuff from the early sixties, and it looks and fires O.K. But it is pushing fifty (so am I if it comes to that) so after I use this up, no more. You can still buy new brass, and I am hoarding it. With neck size only and moderate loads, it should last a good long time. My father brought an Italian Mannlicher-Carcano back from the war. He never would fire it, just hung it on the wall. My father in law had a couple of Arisakas and a Nambu he took off dead Japanese soldiers at Iwo and Guadalcanal. Same thing. Sharpe in his book has very ugly things to say about both these rifles. Recommended that they NEVER be fired (he was not a man to hide his opinions).