Let me preface this by saying that I've got extraordinarily fewer hours in Skippy cars than almost anyone else on this forum, but all of those hours have been at LRP. What happened in that clip looks like something that used to happen all the time to myself and my friends on the gravel rally tracks in Maine.
It looks like you turned in a touch early (although I can hear Bruce in the back of my head saying "Turning early is actually safer at the uphill...") with the effect that you're actually running out of track before you hit the compression and gain grip. Because you need to turn in a much more urgent way, the split second between the front gaining grip and the rear following suit allows the rear to get loose real quick-like.
The specific points on the video that point me to this conclusion are 1)the fact that you had about a foot or so left between your right front wheel and the apex. I got my head torn off by Phil for that one. 2) The car appears to gain grip to the right suddenly in a way that doesn't quite look like the back coming around. It looks like the back comes around as a consequence of the jerk to the right, not that the jerk to the right comes as a consequence of the back jerking to the left.
Once again, I'm not an instructor, nor am I all that terribly experienced in Skippy cars. I just can't help myself...
edit: On subsequent review, I'm going to go with an early turn in and insufficient weight shift to the rear under throttle... The compression may have played a part in avoiding a four wheel slide, but I don't think it was the primary cause. Wouldn't be the first time I was wrong.