I've got a wee bit of a theory on it. Might be completely wrong and am happy to be shot down..
I think sometime around the Hearts pumping, the board told W&W they'd be sacked or resigned at some later date. When Warburton was spotted at Nottingham they saw an opportunity to cut their losses without having to give them a payout.
It could explain Warburton's initial bewilderment when the news broke he had resigned.
Could explain why the statement from the board was confusing as there seemed to be a discrepancy in their timeline of events. Iirc they claimed W&W had resigned on a certain date but they had took training on days after.
Might also explain why they went to the LMA with a grievance.