UPL might be ready to back up Ullmark when Sutton's contract expires in 2021.
Sutton's contract gives UPL the 19/20 & 20/21 seasons in Rochester to acclimate to the pro game.
I'd just heard from multiple places that Buf believes Ullmark is their goalie of the future and with the strong year Hutton is having Buf looked pretty set in net, so I figured they might as well use the asset in UPL to get the team a giant upgrade on the blue line. At least personally I just see a fit in terms of both teams trading from positions of strength to be mutually beneficial.