Just speculating that maybe a prop went bad or was way out of balance and the vibration from that is what helped crack the plastic and dislodge the motor. I say this because the crack surrounds the bolt pattern exactly...
I'm not trying to add fuel to any fire, but in actuality, you could have prevented it by adding the strong arm supports when you bought it like many of us have. This shell failure just shows for a fact that adding the strong arm supports when new makes a huge difference. I'm also assuming that you do balance the props and that one of them failed or lost a tip or something