I'm taking off my artist hat and putting my "Veteran Wooter" hat back on...
I think the Woot.com redesign has a lot to do with the drop in sales.
First (on the strictly shirt side of things)--we have more side sales, which are going to affect first day sales (in theory, folks only have so much money). Not sure the effect is all that significant though.
Second, and really the focus of this post--I'm going with my gut and wager a large percentage of shirt sales come from people who click over to shirt.woot after first going to woot.com. Woot.com's traffic has been dropping pretty steadily since their redesign, based on site analytics [percentage edited because I can't verify accuracy].
Now when we say "redesign," we're really talking about a change to allow many more sales than "one a day," which I think works awesome for shirt.woot, because it offers unique, original products. But Woot.com built its empire on a model where someone could come in, check Woot, say yes/no to the one thing offered, then proceed (hopefully checking wine, shirt, etc. too). It was quick and there wasn't too much work involved. We saw tons of copycat sites based on this model.
Then the redesign came, and that model kind of went away. Suddenly buyers were faced with 8 different Woot sites, all with a main sale and multiple side sales featuring multiple products. Suddenly Woot became work, and it didn't really feel like a sale-a-day site anymore. And a lot of people who used to quickly click over to Woot to see what was up, stopped bothering.
So in the end, I think the loss of Woot traffic has had a large affect on shirt.woot sales through loss of click-through traffic.