My totally uninformed guess is that the announcement was probably not due till Monday but someone in the know has not been discreet or inexperienced in handling the press and the press have got a hold of it, so forced the early announcement?
Otherwise it's bizarre timing, which took the shine off a positive performance by team and fans alike.
But IMO he's done his bit, which we all should be grateful for, but he's taken us as far as he can and was time for him to go.