I am sorry for the lost kiter's wife and son, strength.
I think the uncontrolled kite looping may be what happened, again as speculation. The wind was fairly good at 15 to 20 kts., removing the need for strong sining, absent a small kite. dandaka do you have information about what sized kite he was on? I am not certain when the looping started however. Were there any witnesses at the crowded launch area? It looks like the kite may have been looping when it first appeared leaving the shore whether by intent or accidentally. The lines may be been rigged incorrectly or there could have been a tangle around the time of launch if so? That duration of powered looping could be enough to cause a heart attack by itself.
From the youtube caption:
"В воскресенье 9 апреля 2017 в Зеленоградске (Калининградская область) произошла трагедия, погиб один из наших кайтсерферов "саня111". Я не знал его лично, но судя по отзывам он был храбрым дядькой — катался часто один в суровых условиях. В этот раз он не справился с холодным морем
Остались жена и сын 15 лет. Вытащили его еще с пульсом, скорая ехала 50 минут..."
and poor machine translation:
On Sunday, April 9, 2017 in Zelenogradsk (Kaliningrad region), a tragedy occurred, one of our kitesurfers "sanya111" died. I did not know him personally, but judging by the responses he was a brave uncle - he often skated alone in harsh conditions. This time he did not cope with the cold sea: (Remained wife and son of 15. They pulled him out with a pulse, the ambulance drove 50 minutes ...