With just a quick look it appears to me to be a bending failure across the stress concentration effect of the four holes drilled in the plate to accept the vang lead block. It appears the eye extends into the deck with a slight bend so it appears to me that the design should not have transferred load to the plate from the vang. The vang block is the fairlead back to the cockpit so the eye does not see more than 25% of the vang load, assuming the vang is 4 to 1 purchase. I suspect the eye became loose and transferred some load to the plate in the aft direction. Also the plate may have been installed in an area of uneven deck which could put it in bending load. I think you might consider how much load the deck plate carries in the aft direction and think about whether an adequate fix may be spot welding the piece or even consider an epoxy fix. Perhaps having a rigger inspect the failure and offer other opinions.
Update:
Looking more carefully it appears to me that the bedding of the deck plate, which carries the entire weight of the rig plus the downward force of the rigging, was uneven which may have placed a large bending load on the plate and it failed across the stress concentration effect of the holes. Looking carefully at the aft end of the plate it does appear the bedding is uneven. It appears to me that the plate was never intended to carry a bending load, or a vertical load in way of those holes for the eye bolt and the phillips head bolts. Most likely the mast pad was designed only for vertical compression loads. Also the design of the vang eye bolt was not designed to transfer load to the mast pad. Is the eye bolt loose?
