Last year, he earned an all-star nod as well as having strong numbers of a 2.65 GAA and a .915 sv%. He also makes below the league minimum in salary which is possible because he signed a 3-year deal before the league minimum was raised to $775,000.
The answer is rather simple, the Golden Knights committed to Adin Hill by giving him a massive extension and they have Robin Lehner coming back potentially. While it is unsure if Lehner will ever be 100% again, his return would force the Knights to role three goaltenders, and with one being so cheap, it only makes sense to move him out for other assets that could help them now.