Why was Thornhill/Kaplan's bid of $2000 considered outrageous?

More important than outrageous, it wasn't believed to be genuine

The Auction House and Auctioneer are tasked with selling an item on behalf of the owner, getting the best value they can and taking a fee in the process.

As you say, Thornhill has been deliberately disrupting the proceeding, out there in the open where he cannot be kidnapped or killed, hoping that the police are called.

He's called into question whether the item is genuine, bid strange amounts, underbid $13 instead of $1,300, outbid himself. None of this is the normal behavior at an auction.

Your question is, why does the auction house ignore his $2,000 bid? Well from the auction house perspective, is the bid genuine? Will Thornhill, who has called into question whether the item is fake, be able or willing to pay up? Will the owner or other bidders be annoyed or even threaten to sue if "normal" bids are overlooked because of bids from a disruptive, possible madman in the room.

They elect to ignore Thornhill and call the police to get him out of the auction as soon as possible. They don't believe the $2,000 and subsequent bids of $2,100, $2,500 and $3,000 from him are genuine. After all he said a minute or so before hand that $13 was more than it was worth.

eBay bidding does not work the same way as bidding in the auction.

In an in-person auction, the auctioneer will usually pick some initial price. If a bid is placed, the auctioneer will then increment the price by some amount, and ask if someone is willing to pay that much. This repeats until no one is willing to bid a higher amount, and the auctioneer declares the item sold.

The protocol works this way because buyers don't want to pay more than necessary. Perhaps the buyer that values the item highest values it at $2000, but the buyer that values it second highest values it at $1200. If you were the one that values the item highest, would you rather pay $1300 or $2000? It is irrational to disclose you'd be willing to pay $2000 when you only need to pay more than $1200 to win the auction.

eBay doesn't quite work like this, because when you enter a "bid" you aren't agreeing to pay that much, you are agreeing to pay up to that much. eBay uses a system they call automatic bidding. They reference "setting up" automatic bidding, but as far as I know all eBay bids work this way, and always have, with no action required to "set it up". If you bid $2000 and the next best bid is $1200, you pay $1225, not the full $2000. eBay can work like this because you can enter your bid in a form without shouting it to the entire room where all the other potential buyers, and also quite likely the seller, will hear.

It’s not that it was unreasonable, just given his past behavior it was questionable —- and upon asking for the clarification, he increases his bid again and again. Raising the bid by a large amount is something that happens in auctions, it is a way to signal to others that they shouldn’t waste time by bidding against you, you are determined to get it and can afford it. But upon being questioned he raises his bid again and then yet again. At that point they decide that he simply isn’t being serious.

Because raising the bid again and again without a counter bid is NOT how an auction works.

Which is a long way of saying that it wasn’t the 2k bid that ultimately was the problem, it was his reaction to being questioned about it that was unreasonable.