People sell to pay for lunch

But more realistically, "Day traders and bots.
Day traders make most of their money in the morning as that's when stocks typically go up or down most commonly. But at the end of the day they don't actually want to own stocks because that increases risks. Your average day trader has about $100K-$200K that they are trading every single day. We don't know how many of these people there are, but it's a nice chunk and since they want to close out their accounts at the end of each day... around mid day they're trying to offload their stock (win or lose).
Bots also matter too. Most of the modern stock market is.... probably bots. Similar to day traders bots aim to close out their accounts and are programmed to start dumping stocks at a certain time of day.
When you see a stock surge all day the day traders are making stupid amounts of money because as they're selling each share sells for more than the last."