I see nothing in that picture that suggests the case mouth did not open properly. What I see is a curling in, on the two corners adjacent to the tear, where the piece of the hull is missing. That shearing could very well cause the curling.

Not that it means much either way. We'll likely never know the cause, but I can easily see how, if the chamber wall gave way first, the shell casing would be ripped in this way.

SRH

May God bless America and those who defend her.