I would guess they have poor power, low freeboard when loaded, poor seahandling character. I would guess they are fine for a river crossing or a beach landing running with the waves. I would guess they would take water over the bow heading into rollers. They are open, so they fill with water and sink.
We all know squalls can blow up suddenly. The driver probably tried to point upwind, took waves over the bow til it sunk. I am not sure whether he could run with the wind. Maybe too slow or heavy. Get pooped. What happens when he gets to the lee shore? In the best case, he drives up on it. Or he takes waves over the stern and sinks? It is a vehicle that does it all and none of it well.
A complex of events ending in tragedy.