Apparently Jessica made a request to halt her promotions with the group after their next album, meaning she would stick around for one more release and then leave to focus on her individual career. SM on the other hand were not having it, probably finding her proposal unreasonable and are instead making her leave right here right now. The words posted on Jessica's behalf are hurt and disappointed, and reading those certainly didn't make it seem like she wanted to leave the group anytime soon, but had to because she couldn't compromise with her agency. I do see a big problem with thie issue in general, and I think that if Jessica wanted to leave after the next promotional round she shouldn't have signed the contract at all. I'm not sure if letting her go now is the solution either though, but I do know that SNSD will take a huge blow to their image and loose a lot of fans.
I don't necessarily blame Jessica, or any other of the members, for focusing on their careers outside of SNSD because the idol life is only so long, and most of us knew that they've already entered the final stages of group activities. As news of the renewal of their contract for another 3 years came out, the overall feeling was that it would be over when those years had passed, as the girls would be in their mid- to late twenties and have established personal careers and lives by then. My assumption is that Jessica wanted to combine her singing career with her fashion career, and SM wasn't going to let that happen simultaneously. Instead they've kicked her out of the group because that's the only solution they could reach, which is frustrating for everyone except the agency itself since Jessica is still signed to them but as an individual.
My honest opinion is that if they weren't going to let her participate as she wanted to they should have terminated the contract entirely. As the situation is now, she's still with SM but isn't allowed to be a part of the group, and that must feel terrible since that probably wasn't her original plan. This entire situation is chaotic and messy and I believe SM have really screwed things up this time around. I can for sure say that I wouldn't be surprised if the whole group announced their disbandment anytime soon, because the end feels scarily near.