The classic way to get VRS is on a downwind approach - as he is.
Then let yourself get steep - as he does.
Then start to try to recover with power - as he does.
Then try to reduce speed with a flare that won’t have any effect due to being downwind - as he does.
Then experience what some long-line pilots describe as falling into a hole - as he does.
And finally crash because you don’t have enough height for recovery - as he does.
You need a RoD between 0.5 and 0.75 approx of your down wash speed to encounter VRS with a forward airspeed of less than about 15 Kts - the lighter the aircraft the lower the RoD required as the downwash speed is lower.
He may not have got into fully developed VRS but I see no indication of over pitching or settling with power.
I remain with VRS as the cause.