Sounds like the guy spend an awfully long time in the water being only 100 yds from shore. I think I may have beaten Phelps' world record getting the F out of there. Terrifying, but do you think the shark was just playing around like a puppy with a chew toy?
Probably a few things... the shock getting dunked unexpectedly in low 50 degree water shark or no shark, he had the proper clothes but no booties; Was wearing a pdf, hard to swim in that; from his account he's really not sure how long it actually was, understandably; don't really want to be thrashing around diverting the shark's attention from the kayak.
No idea what was in the shark's head, but as usual if it wanted to "attack" him, he wouldn't be here to talk about it likely. From what I know, and so FWIW, sharks "feel" by their teeth. Likely it was curious as to what the boat was, or perhaps thought it was prey from below hence the approach from underneath, which is how they hunt (although at 14' the kayak would be a pretty big prey, so maybe not likely).