That's the magic of interest rates. Countries in the EU, let's say France for example have roughly 115% of GDP of debt. To service the interest of the debt it must finance each year the debt by paying the interests, and borrowing the sum on the market to reimburse the previous debts which are currently reaching their terms. The full owed amount is never paid back, but can be rolled forward indefinitely.

These interests are currently ~2% for France. Which mean the debt is manageable and the interests can be paid with the citizen's tax and the music can continue to play. But once France get out of the UE, interests rates become 5% then the citizens tax are not enough to pay the debt, and nobody wants to lend money to France anymore because even at 5% interests the risk of default becomes too great and they risk not getting the full amount-owed back so nobody lends, and since their is no money in reserve, and they can't borrow it means they default => bankruptcy. France doesn't have its own currency anymore so it cannot print its own money which compounds the problem. National resources get plundered, citizens get poor.

It is a game of musical chair which is highly non-linear.