He didn't pay his taxes when he should have. The IRS doesn't require you to withhold income and/or capital gains when you take the money. Although every major broker gives you a gentle suggestion that you should every time you take out cash. So in a bull market like this, if you're not withholding, you could very easily make a bunch of money, not pay or defer taxes until later and then not have the money when you owe it. There's an easy solution, just withhold as you take the money out. But people don't like the IRS having their money so they say "I'm not getting a refund and giving them free interest. I'll just pay it when I owe it." And then the inevitable happens.
At least you're not like me and owing irs about 20k....but also having a blown account 🥺 lol I can't pay that shit from my job income. I fawked up y'all.