Because somebody bid in the last few seconds, and the clock gave the room a chance to answer.
What it is for
On a fixed timer, the winning strategy is to say nothing until the final second. That turns an auction into a reflex test — the fastest connection wins rather than the highest offer — and it leaves the seller with whatever the clock happened to catch.
The extension removes the advantage. There is no point waiting, because waiting does not end it.
What it is not
It is not a trick to keep you bidding. The clock stops as soon as people stop — it only moves when a new bid lands, and the price only rises because somebody chose to raise it.
As a bidder
It means you are never cut off mid-decision. If you are outbid at the death, you have time to answer. Decide your number before the item comes up, and let the extension work for you rather than letting it pull you past it.
Except on sudden death
Not every item works this way. A seller picks one of three formats, and on sudden death the clock is short and never moves — the highest bid when it hits zero wins, and waiting until the last second is a real strategy rather than a futile one.
The item says which format it is. If it is sudden death, treat the clock as final.
Questions people ask
How long does it extend by?
Enough for the room to answer. It is short, and it only happens when a bid actually lands late.
Can it go on forever?
Only if people keep bidding, which means the price keeps rising. In practice bidding stops.
Does every item work this way?
No. Sudden-death items have a short clock that never moves. The item says which format it is.
Does it apply to buy-now items?
No. A fixed-price item ends the moment somebody takes it.
More: how bidding works, or the ten seconds after a bid.