I think one of two things is happening:
1) Walker will be traded at the deadline. He doesn't fit how Will wants to play with a five-out offense and swarming, switching defenses. They're playing things light and mum to keep trade value as high as it can be. This would explain Walker's frustration and that, in the next sentence, he expressed a desire to be a part of what the Jazz are building.
2) More exciting but less probable: there's someone/something on the Jazz's radar for this summer and the Ainges are trying to pull off the most legendary accomplishment for an NBA GM: a two-window team. What did Ainge do after drafting Tatum and Brown? He brought in Hayward and Kyrie. If we have Ace and AJ/Booz/Peterson, all of our young talent, Lauri and a maxxed out All-Star or All-NBA player... that's a two window team. By not extending Kessler, they could bring in the max salary and then sign Kessler to a new contract into the apron. Doesn't seem to be a great free agent class, so if this is what pans out it may be via trade.
(Personally, I think it's number 1, but wouldn't be surprised with number 2. Ainge works this **** out well in advance - the Celtic baby onesies weren't an accident; he spent 3 years with KG as a target for Boston.)