They get a credit card. Credit makes it easy to spend money one does not have. Students often buy things without hesitation that they wouldn't if they had to pay cash. In less than a blink of an eye, a person can make purchases way beyond their ability to pay. At first, the low payment sounds enticing.
Most students figure they can afford twenty or so dollars a month.
It is not an economical way to borrow. Another big danger of runaway credit card debt is establishing a poor credit history. That information is used to indicate a poor credit history that can be passed onto anyone from whom the cardholder might want credit, such as banks, stores, and even landlords. A poor credit history will follow a person everywhere. It is almost impossible to get rid of it.