It was an 8cm crack (almost half the depth of the speaker) caving in part of the casing. It looks like it was dropped on a post with a rubber cap or something as there was no puncture wound per se (other than the carton), just impact damage.
From my experience, I would say something landed on it, as the M3 is not that hard to handle. The chutes the packages go down get clogged up and stuff up to 70lbs will go down them. The box doesn't stand a chance under the right circumstances. Some of the things I have seen go down the lines should never go.