Buoyancy, thats the short answer at least. Clearly went deep, when he was sucked out probably less than 1/2 his boat filled with water and then it shot out of there like it was its job. The deeper the kayak goes before the skirt implosion, the larger the bouyant force pushing towards the surface on the boat will be. Part of him swimming could possibly be a result of his momentum still taking him a bit deeper and the boat accelerating toward the surface causing him to somersault out.
Pretty much like this video, minus the guy staying in the boat.
Or, I'm all wrong, wouldn't be the first time. I do push rubber after all.