I remember this now - I was diving the week after on the Porthkerris hardboat. The boat went down somewhere between the manacles and porthoustock, just round the corner from porthkerris. The skipper had been up on the cliffs looking for a petrol slick as he was after the engine. From what I remember, he said it was a dorey rather than a rib, but from the dates I'm pretty sure they are the same incident. The boat had come round from lands end, across mounts bay and round the lizard.
The porthkerris people might have some more info.
If it was a rib, then I guess they damaged the hull at the rear, getting water into the hull cavity and through the transom. The weight of the engine could pull it down then, especially if there was extra kit at the stern. If the sponson was not enough to keep the boat afloat, then the pressure on the bottom of the tubes at 5-10m underwater could be too much for it, hence the burst.