I enjoy these types of questions, so thank you for posting.
While there are probably too many things to consider for one of us that was not there to "answer" your question maybe I can suggest a few things that could have increased the chance of oversteer.
1) The "ruts" in the road, no matter how slight are there. If you were transitioning from the hump (both wheels on the crown of the ruts) down into the ruts, the wheels would lose weight briefly and thus lose traction. Once the oversteer carries the contact patch to the base of the rut, you would gain traction as the wheel is trying to move up hill.
2) Road grime is not equally distributed. You will find more in the crowns of these ruts than you will find in the ruts themselve as the constant tires running through them will push it out of the way.
3) Throttle changes mid turn, but I doubt it if you have "Proskills"
and were in 4th at 50 mph.
4) Pavement shift, dip, or change in material. If there were a dip, or crack this could cause it. Also if there was a patch of new pavement holding you and then it changed to old pavement. Also, if there was a patch of concrete vs pavement, that would also reduce the traction.
Maybe one of those helps?
Thanks guys, I think my tires are at first wear, but they definitely have some tread left. The tires were very hot at the time, I had just come up from San Diego. I assume that my rears broke traction when I accelerated off the apex. I think it just spooked me because I have never lost traction off the track. :/
From what I remember, I was low rpm in 4th, so maybe around 50 mph (never shifted or braked). I have done similar off/on ramps at this speed or faster without ever losing my tires...