They saw what happened with D-Law, and it'll happen again.
Contracts always go up. What seemed like an "overpayment" a year ago is now considered ok the year after and 2 years later it's a steal.
It's like the rising house prices but without the fear of defaulting on your loan.
Say Dak wants 32 mil this year but the Cowboys are stuck on 30.5mil, for example. He goes out and has a good year, posts better numbers etc... He isn't going to accept 32 mil in 2020, and may want closer to 34-35, especially since Mahomes will be eligible for an extension in 2020 and he'll definitely reset the market.