Firstly, thank you for the question & it is an interesting one.
In short & hopefully simple. Manchester United are preoccupied on paying their debts to the banks that they owed to. Also, as they borrow means they will have to pay interest. For example, if eating a meal at a restaurant cost say $5. We would normally have to pay say $5.50 where $0.50 is the tip for the waiter.
Similarly, the interest behave roughly the same as the tip relating to the above example. Thus, as Manchester United delay in paying their borrowed amount (loan). They will have to pay more interest because the interest keeps on multiplying as the months or years gone by.
So, the more delays they do, the more interest as the rates changes relating to the country's economy situation.
http://en.wikipedia.org/wiki/Glazer_own ... ter_United
Hope that helps