The revenue from the lackluster NBA playoffs came in much lower than projected, dropping next season's salary cap. Every team having $3M less now then projected could have a decent ripple effect throughout free agency.
The Celtics now will have to clear some cap space to sign Hayward. I think this really affects George Hill too. I just don't see him getting his $25M/year. I think in the end the Jazz's offer last February may have netted him more money ($89M over the next 3 years, but $13M of that was backloading this year's salary.) I think he turned it down in part because he wants to get that 4th year guaranteed as well for when he's 35 years old.