Or the foil could have struck one of the numerous semi-submerged hunks of crap often seen in the bay. (as was suggested by an earlier poster)Nobody knows yet. The videos may not clearly show the cause either, just the aftermath.
That was mentioned in the UK Guardian article by Fisher
Foil failure/grounding is not only unlikely it's irrelevant. The boat should survive a PP intact as per OR. The strength of the beam is probably also irrelevant as not only would it have been load tested but in the event of a rigging failure it's going to break, in bending, at that point. With the stay intact first mode of failure is either compression or buckling, neither of which would look like that.
Also as it appears that the salvors have removed the bobstsay at grand central to facilitate the operation you would assume that if the broken port hull were still attached via the stay that they would have removed the pin from that too.