ok, I am no adjuster and not sure you will find one that can say what happened.
There are lots of things that can cause a boat to sink.
The most obvious one is something that lets water in.
But, if your saying it floated afterwards and there is no leaks, then we have to look to something from above.
Bilge is working, so rain water would of been pumped out
What about ropes?
Was boat tied properly?
I once saw a guy tie his rope up high around a pole, rope slid down pole to below water surface at low tide, high tide came up, rope held boat down to the point where water started running in thru the thru holes. Eventually it overtakes what the bilge can do or bilge dies if just on battery.
I would not be to quick to say sabotoge and besides, insurance should cover it either way!