A classic catapult triggered by the sudden (relative) breaking resistance of the board (caused by a gust, or submerging the nose in the back of a wave, or cutting the leeward rail down in the water, or ventilating the fin,

or ...).

 

Traveling in approx. 13 m/s cross offshore wind on a Tiga Freecarve 61 with a Gaastra  Grind 5.4.
Anders is happily planing on a beam reach - but what you can't see from these awful pictures, is that he has in fact reason to feel a little stressed by his sailing buddy, who threatens to overtake him from a leeward position (see the original video here). He is pushing hard - and perhaps a little too hard. His more inboard/forward position and the lifting of the clew of his sail indicates, that for whatever reason (perhaps a spin out - or perhaps running into the back of a little steep wave) the board isn't traveling with the speed of Anders and his rig anymore.
The nose of the board is placed low in the water, and now it also looks like the board is capsizing a little to the leeward. The board is definitely trying to stop Anders, and in his effort to stop his movement forward he clings to the boom and over-sheets the sail.
The board now seems to have transformed into a submarine, and Anders - still tied to the rig through the harness line - pulls the rig forward.

The 100 kg weight of Anders slams into the water with his heaviest parts first (belly and arse) - pulling the rig in the same direction.

The downward pressure on the board has been so great that it now pops up in the air behind Anders.
Is this whales breaching or tail slapping? Well, if it wasn't for the blurred pictures above, we wouldn't know.  A couple of minutes later: Still with the life jacket high up his neck Anders attempts to water start. But look at that asymmetric boom - you can almost hear the local windsurf-pusher laughing ...