If you cut through all the BS, finger-pointing (both ways), rumor, and speculation, I think it's as simple as Nesta realized he was probably not going to start this year. He was going to be the 3rd or 4th DT IMO.
JHH, Taylor, and a portal guy (or two) were all going to unseat him. I don't think he was incredibly productive, plateaued, and is a guy that is more indicative of what the talent was where we were, not where we are going.
That I all believe. If I were to speculate, I'd wonder if he was scared off by the 4th Q program as well. I get not wanting to risk his money season by losing reps.
My thoughts.