With Murphy back and Boqvist up, the Hawks would have too many contracts with bonus structures. The CBA doesn't allow for you to exceed more than 7.5% of the cap in total bonuses. If you exceed the bonus cushion, you exceed the Cap. Between Strome, Kubalik, Dach, Nylander and DeBrincat they had enough potential bonuses that they couldn't keep Boqvist up legally.
Math:
6.71M in potential bonuses without Boqvist-1.27M in capspace=5.44M in overages
7.5% of cap=6.11M
5.44M+850k=6.29M which is over the cushion.
They literally can't have Boqvist up unless someone is on LTIR this season. It would be illegal otherwise.