The big mistake this guy made was "powering up" his kite (increasing
angle of attack) to jump over some kite lines on the beach. Jumping on
land is a major no-no because if you get lofted, the landing hurts
because it ends with a splat, not a splash. In this case it was not
just a simple lofting, because he got caught in a thermal inflow.

If you watch the video carefully, you can see that he is getting sucked
into the core of the thermal under the dark cloud as he climbs
initially. Then he gets spit out the side, and descends rapidly,
presumably in the downflow around the thermal.

The kite he is flying is probably around 16m^2 or around 170 ft^2.
Assuming the guy weighs around 170 lbs or so, that's a wing loading of
only about 1 lb/ft^2.