It did look as if Westwood made an impulsive last-minute or heat-of-the-moment-style attempt to take the attacker out once contact with the attacker became inevitable. Obviously, it is extremely likely that the attacker would have run around him and scored otherwise. Westwood probably knew that's how the incident had looked as soon as he brought the attacker down and, from observing his immediate (and pained) reaction, he clearly regretted his rashness; he's cringing with his head in his hands, in the shape of a human ball, desperately wanting the ground to swallow him up. He's probably expecting a red card to be waving in his face when he decides he's able to return to reality and face the expected consequences. As it turned out, he was only given a booking, probably much to his surprise and relief. This may well have given him the impetus and motivation to make that extra effort and save the penalty.
To try and work out why the ref decided that a yellow was appropriate in spite of how the challenge appeared; Westwood was already committed to a challenge or engagement of some sort and it's probably fair to say his original intent was to win the ball or to at least cover the goal from it (both of which could surely be construed as "attempting to play the ball" rather than the man). As a result, the momentum caused by this inevitably drew him in, so maybe the referee felt he was trying to play the ball on that basis. That's pure speculation (with a healthy dollop of amateur psychology too) on my part though! :)