I've seen this happen, and it's very frustrating for the one holding the auction: The highest bidder wins the auction, and once it comes time for them to pay up, they go "Oh nevermind I didn't want it anyways".
Some people bid a lot on an adoptable and end up not paying. Which is what 'fake' bids are. A lot of people do it just because they think its funny to get peoples hopes up, when all it does is insult the artist and their work.
That could also be the case, but usually what happens is that people sometimes forget they are in an auction, and when they win, they sometimes get scared and not say anything for days, in fear of angering the artist.
There are many situations that could have happened for people not to pay, but in the end, if others are not sure enough to hold up to their bid, they shouldn't have risked it in the first place. ouo