Too much conjecture here. The club have failed to give him the 'dreaded vote of confidence in public' , that in the footballing world normally equates to the board are happy enough with him.
The board aren't backing his style of football yet have invested in Jorginho, brought Higuain in ( apparently against their better judgement) and invested in a world record fee for a keeper he coveted whilst at Napoli. We are currently under a transfer ban but who knows what the plans are if we were to get a stay of execution, our board quite rightly like to keep their transfer targets close to their chest.
The board are allowing him to leave for another job, there is no use keeping a manager at the club who has had his head turned.
Just possibly, and this is conjecture too. Sarri has found adapting his life both professional and family to a new country difficult. He has a good job offer back in his native Italy with one of the biggest clubs in the world and has spoken to the board, who may just have decided to let him leave, having achieved his goals for the season. All parties walk away happy.