Two reasons, David.
Firstly he has been badge kissing and playing up to the fans for some time now. You'll remember that he jumped up onto the East Stand barrier wall for example. He also said that he liked it at Motherwell. Now I'm not against anyone celebrating when he scores a goal for us, but these actions were more theatrical than most. The first time that news broke publicly about him turning down our offers and stating his intention to move on was 10 December. Talks had apparently broken down. Now these talks don't just start and conclude in a day or so they can take weeks or even months. So while he was playing to the fans its likely that he had decided to move on at the end of the season. In short he built up unrealistic expectations. He would have been better advised to tone down the rapport with the punters. In fairness, he's not the first nor will he be the last to do this.
Secondly, the leak about moving to Dundee United. This occurred on Christmas Eve, just before we were due to play our final game before the break. Very odd. Now he could have been negotiating a move with United since 1 November and thats perfectly fine. Someone leaked news of the PCA to the press. Now that could only have been himself, his agent (surely acting on his permission), Dundee United or "a source close to". This has been fairly widely publicised in the press most notably by the Courier. There was no need whatsoever for this, unless it was designed to instigate a move next month. Many clubs and players sign players on PCAs, including us, but very few publicise them so well in advance unless they're trying to pressurise the selling club into an early move. Not professional or courteous in my view. Had the leak come from the player or his agent, without the approval of United it would have been very risky indeed. How would he know that United had the cash to sign him next month? He's bound to have known the fallout it would cause at Fir Park and if he didn't his agent certainly would know. It might have caused a rift with United. No, the most likely explanation is that it came from United who used the press to their advantage. Now its no secret they want to strengthen their striking department next month. Once the story had been leaked to a compliant journo before the window opened United, Watt and his agent could afford to lie low and keep quiet - job done. The leak had achieved its purpose, by pressurising our club.
I'm sure there's much more to this than I know and I may have made false assumptions but thats my take on it.