I've had one piece of Hornet brass separate as well, after loading them with 12.2grs of H4227.
The problem was, 10.2grs of H4227 was the max load. I mis-read my balance beam scale.
I used a thread tap as well. The separation happened way back in 1990, or so.
Since Lilgun powder and a Lee Collet Die, I have not had another one.