In the case of Harper, he had made a public announcement stating that he had requested his release during WrestleMania 35 season, only to have it rejected. Since then, he has been in the dog house with WWE. However, the company finally decided to grant his release recently.
As for Sin Cara, he had oddly requested his release shortly after signing a new 3-year contract with the company. While it initially appeared to be a similar situation as that of Harper, the Luchador was released as well.
From the looks of things, WWE is finally beginning to grant certain underutilized Superstars their release. It's the best move without question since the roster is already stacked with talent across the three brands.
According to WON, the releases aren't going to stop just yet. They noted that the age of each of the Superstars released was above 39 except for Jordan Myles/ACH.
Regarding releases, within the company the belief is that there are more coming. But these things change rapidly, as for so long it was nobody getting released, then it was if they aren’t happy let them go, and then still, nobody was getting released. It is notable that everyone released was 39 or older except for Albert Hardie Jr. (ACH/Jordan Myles)
It's an interesting move and perhaps for the best. In 2019, there are a lot of options as a pro wrestler aside from just AEW. Many Superstars such as Cody Rhodes, Enzo Amore, TJP, etc. all stated that working in the independents is now more financially rewarding than working in WWE.
Moreover, they have better control of their schedules so it appears to be a win-win situation.