They need to sell the day before settlement is due generally....

They need to sell the day before settlement is due generally. ASX operates on T+2 for now, which means you can buy shares without money in your account, so long as you have the required money in your settlement account two business days later or make a contra trade prior to two days later. So what this means is you get traders buying shares with the expectation that it will go up that day, or the following day, and they can then sell those same shares to cover their costs + make some profits. They need to sell them because they likely don't have the full amount to cover the purchase as they're using the T+2 like an extremely short loan for leverage.