Billionaires' favorite tax trick of borrowing against their stocks is backfiring, and the fallout is spilling into everyday retirement accounts and the housing market.

• Rich people avoid taxes by never selling stock — instead they borrow cash against it, but these are "demand loans" banks can call back anytime.
• Borrowing costs tied to those loans jumped sharply as interest rates rose, and when a stock drops even a little, automated systems force a quick sale with no human involved.
• Those forced sales dump shares of big companies like Apple and Nvidia onto the market, which hits the same stocks sitting inside ordinary 401(k)s and pension funds.
• Total borrowing against stock portfolios has surged, cash reserves are at record lows, and one month saw the largest sudden payback of margin debt ever recorded, a sign of forced selling rather than people paying down debt by choice.
• The same squeeze is hammering commercial real estate, with office towers selling for less than half their old price, which is making banks pull back on loans to small businesses and regular homebuyers.

Outlook: Expect more of these forced sales to hit stocks and buildings gradually rather than all at once, with regular investors and homebuyers absorbing losses from a debt problem they never signed up for.