As you say, we don’t know what may or may not have been said.
Maybe the owner simply felt it was time for a change? Or is unhappy with the jockey situation at Ditcheat with Cobden departing?
Or it could just be a case of the owner wanting to be with the most successful trainer, which Nicholls was when he first had horses with him but he isn’t now.
De la Hey isn’t the first high profile owner Nicholls has lost recently. He will still have a few decent horses via McNeill, Ferguson/Mason etc. But it is very difficult to see him getting back to the level he was at before.