Why oil traders selling oil on the day before expire date but not the other days earlier than that?

They weren't forced to sell on that particular day - in fact they could have sold the next day (on the day of expiry), or they could have sold the day before. But they were facing a ticking clock and as the market started to move down some were apparently willing to get rid of these contracts at any price. Those that had the guts to wait until the next day were able to sell them for up to $9, but no one know the day before whether the price would rebound or not. It's not necessarily easier to sell on any particular day.

Traders of futures contracts, not taking delivery and not making delivery, should move to the next dated contract at least when that contract has more volume than the current contract.

And ETF's move to the next contract on a calendar schedule.

So who held on ?

A buy-side wouldn't be inclined to hold-on unless they could take delivery. The oil had more value than the value of the contract and so just take delivery. Well they might sell looking to re-buy delivery at a lower cost a few hours later.

A sell-side could be inclined to hold-on because of the extreme contango value even if not making delivery. But the sell-side has to eventually buy to get out if they are not making delivery.

It looks like the sell-side was making delivery and so not buying. And the buy-side was taking delivery but tried to get a cheaper contract by selling and re-buying.

It looks like an industry oil-trader panic and not a speculator panic. And the situation could have been a low volume pricing.

The buy-side might have bought more to average their price but oil storage was limited.

Oh, I didn't consider the crash to be a market-maker flash-crash because the traders still in the contract should have been those taking or making delivery. But the market-makers could have just left the market because the futures market is ultimately based on an accounting of delivery.