Sounds like classic case head separation from over-sizing brass(setting shoulder back too much.
You should be able to just put side pressure on the piece of case and get it loose from underneath the extractor.
I would junk all that brass as its all suspect now,although the load seems fine.
Yes I try to make myself always wear shooting glasses as I have had a similar blow out with factory new ammo but alas when hunting I rarely do so.
"Anyone taking up handloading necessarily plays with unknown factors and takes chances. But so does anyone who drives a car,goes to a cocktail party,eats in a restaurant,or gets married."
Jack O'Connor 1963