I suspect his press conference this week was the thing that finally spurred them into action. Riza looked terrible, talked of splits in the club and branded fans clueless. It was as if he was seeking the sack.
I reckon Tan and Co thought it was too late to do anything before the Sheffield game as the team was already travelling to that and they will have expected to lose anyway. They probably talked to Ramsey and Gunter following the press conference. I doubt they did that this morning (although you never know).