Either that brass one piece of brass is longer, so much so that it's making contact with the crimp feature inside the die,
or, some how some way the seating die got threaded down lower while seating,
or, a combination of the two.
although it's never happened to me, I suppose if a case mouth isn't reamed and chamfered enough, the bullet could hang up on the lip, thus shoving the shoulder down, instead of the bullet entering the mouth. But that one looks really very pronounced, such as what one might expect with an improperly adjusted seating die.
Another possible, is that if the seating die is adjusted down to the point that you only have a few thousandths of gap between the case mouth and the crimp shoulder in the die, then a piece of brass that's longer, or long enough to make hard contact with the crimp feature could certainly cause the shoulder to collapse like that.
I know those who are new to reloading initially think seating depth is adjusted by threading the die down deeper, but that is incorrect, seating depth is adjusted by threading the seating stem down, thus avoiding contact with the crimp shoulder in the die.
GS