The last I heard is that they think they hit something.
But there is a correction, the boat did not completely sink. The bow remained sticking out of the water and on Thursday they were attaching air bags to the hull to try to refloat the boat. As far as attaching it to another boat, they actually considered tying up to two other boats as they were taking on water but were concerned that between the weight of the boat and the wave action, the other two boats could be damaged.
There was an Ocean Yacht Sport Fish (Can't be sure but I think it was either a 46 or a 55) that went down in the tournamant about ten years ago. The fiberglass weakened around one of the propshaft supports, the shaft came loose, the shaft wrapped around the other shaft and tore a rather large whole in the bottom. It went to the ocean floor rather quickly.