This is a bit puzzling.
Seems like declining fertility or some other physical problem is the usual reason behind a 'private' stud fee with popular stallions. His live foal percentages look like they are holding steady at 70%-80% and I've not heard of any health problems or rumors that he didn't settle mares this season.
His auction reports don't look that great to me, but I'm still learning about this so maybe not.
Bloodhorse Stallion Register
11 offered, 7 sold, $1,913,475, avg $273,354, $160,000 med
3 colts, avg $470,000; 4 fillies, avg $125,869
Perhaps Ashford felt that the could no longer command 300K per season but felt it would reflect badly on their flagship stallion to reduce it? They sure made cuts with the other stallions.
Could this 'private' fee be a smoke screen meant to obscure the fact that Ashford has doubts that GC can continue to get 300k per season from 190+/- mares? Maintaining the stud fee and risking that his book will drop would be risky for them, too. Now that he is expected to cover this number of mares, seeing a smaller book at the same price will no doubt make breeders and buyers nervous...
He sure is a nice stallion, but I think putting him at 300k so early in his career is likely to backfire. Many of the mares that he got at this outrageous price were sent by highrolling, speculative breeders that represent a very fickle population. No surprise that that the mares and the money haved moved on to the next prospect and left Giant's Causeway looking a little sad and lonely.