Why are people surprised about this? Poch had done all he could with Spurs. Kept them in the top four season after season and also took them to the last Champs League final.
Call it sacking, departure by mutual consent, whatever... Some things just come to an end eventually. Unfortunately, it had to happen this way.
Guardiola had to leave Barca because of issues with the board and also inability to further motivate the Barca players. Sometimes you reach the end of the road and you need a fresh challenge. Happens in any and every professional career. This should not be a taint on Pochettino's ability as a manager as he is still a good manager who will excel in his next destination.
And this is the reason I consider Sir Alex the best manager ever. The ability to reinvent and alter approaches, keep motivating a bunch of serial winners, see of Abramovich's and Mansour's challenges and not crumble is to be used as a case study in 'how to remain a winner at the top' . Keeping the success train flying is easier than getting one.