Wold the trap you were shooting into happen to be made out of metal? If yes then a piece of the plating may have back spattered causing the exta hole. If it is a wood trap, or has some kind of filler then it could have been from displaced filler, or wood debris. Just a couple of thoughts on other things. Do you still have the target? If so check to see if it looks like the tear is from the other side.
If not since it was your first shot then the round may have separated from the plating. This can be casued by either too hard of a crimp, a deep gouge in the plating, or too hot of a load.
Unless you tumble after sizing/depriming then it would not be possible to deprime the brass with another case inside. It would have jammed up in the die.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.