All the ones at Maghen except for this one are gone as well, and it's not a bad cover at all, especially for the price:
http://www.danielmaghen.com/fr/charlie-adlard-walking-dead-cover-issue-89_p41176.htm
I think a lot of the driving force behind the covers selling is a tipping point aspect - people have their eyes on one, it sells, leading them to move. Some of Adlard's covers wax and wane, but I wouldn't say the ones selling are bad ones by any means.
It will be interesting to see how the cover to 93 does. It has a small art space, has partially printed bluelines, and minimal character placement, but it already has 4 bids on it at the 800 mark (300 above the price on splash page from what I remember).