If a person has a lot of debt and no savings, and they lose their job, then they will find themselves in trouble a lot faster than someone who owns their home and car and has 6 months in a savings account.
If a company has a lot of debt, they might find themselves in trouble with credit rating agencies as soon as they have a bad quarter. Or they might have cashflow issues if rates increase. Both of these can lead to an accelerating negative feedback loop.
Governments (at least those with fiscal independence) have a unique set of tools to work around this situation, but they too can struggle with high debt loads acting as a drag on future prosperity.
Debt plays a critical societal role in allowing new production in advance of revenues, but it can also be a dangerous trap.
These AI and tech companies are priced as if they are still running a capital-light, 0-marginal-cost SaaS business. That's no longer what's happening. This economic engine makes up a substantial amount of both the value and the growth in the American stock market.
If there's a loss in confidence, high leverage will make things fall faster. This could be infectious. Not just the AI and tech companies, but the whole market might suffer.