Uninformed speculation -- it's a function of the players' talent and the needs of teams. Perhaps also a function of whether players have had legitimate opportunities to showcase their potential with their former teams. So much of it is just market dynamics...if the guys can impress beyond just being depth contributors, word will definitely get around. The players still need to make it past the roster cuts to the 53-man roster this fall though...
(Quick guide to how teams get down to 53 players:
https://www.arrowheadpride.com/2024/8/25...procedures)
Speaking of multi-year deals (but not NFL), Quenton Meeks signed a two-year deal with the Montreal Alouettes.