The obvious answer would be that more people put in to be drawn there. Undersubscribed tags are just the leftovers from the draw applications, and really mean nothing other than the amount of people wanting to hunt there has changed.
The number of tags that were available in the first place, before the draw process, tells whether the Game folks think the population is up or down.
Check the stats for previous years, and see how many tags total there were available, how many guys applied, and what priority they had to be to have a chance to be drawn. The stats are public info, and I've used them in the past, to determine where to be putting in my draws.
Cheers
Trev