The article goes on to say that they had $5B net wealth around the time of the incident. It's not that unreasonable to get a loan of 20% of your wealth in a hurry, especially if said loan immediately benefits the lenders.

Seems that in the end it didn't work out well for them:

> The Hunts lost over a billion dollars through this incident, but the family fortunes survived. They pledged most of their assets, including their stake in Placid Oil, as collateral for the rescue loan package they obtained. However, the value of their assets (mainly holdings in oil, sugar, and real estate) declined steadily during the 1980s, and their estimated net wealth declined from $5 billion in 1980 to less than $1 billion in 1988.