A bid being retracted "by eBay" is pretty unusual, but I can think of a couple of circumstances.
The first is that the bidder became NARU, Not A Registered User. This usually happens because the bidder has done something (or several things) BAAAD. Like getting "too many" Unpaid Item Strikes, or going on a bidding spree, bidding on hundreds of items.
The second is that the bidder was Blocked by the seller,either specifically or under his Seller Preferences, and was bidding with a different ID. The eBay 'bots are sometimes a bit slow to catch this one.
Or the bidder could have retracted his bid and lied about the reason. While this is naughty, as a seller I wish there were more reasons for retraction, including "I bid while I was drunk". "I didn't read the description" and "I changed my mind", since from the seller's point of view, why go through the drama of trying to be paid by an unhappy buyer, when I can sell, for a slightly lower price , to a happy buyer.