Maybe the escrow agent for his mortgage forgot to pay? That's what usually happens.
That's possible. Somehow I think it's unlikely that Sullivan can't afford the back taxes. Just thought it was interesting.
By the way, here is the link again (previous line wasn;t converted to hyperlink) :
http://www.gawker.com/news/media/andrew-sullivan/index.php#andrew-sullivan-in-foreclosure-in-provincetown-026790