Yes, it's due to the charges.
But there are ways around this. You could look for a money transfer credit card but they tend to attract a fee.
How much is the balloon payment? One option would be to apply for a 0% purchase card if you don't have one already, Moneysaving Expert shows several available, with long 0% periods.
https://www.moneysavingexpert.com/credit-cards/best-0-credit-cards/#what
So if you use that card for other normal spending (eg groceries, fuel, meals out, Christmas shopping, holidays, etc etc, any non DD spending really) to the amount equivalent to the balloon payment, then just use the money not spent on these things to pay the balloon payment and you've done what you wanted to do, without any fees or interest.
If you start now, you have at least a couple of months head start so might be able to do it, depending on the size of the payment, especially if this is an interest minimisation exercise rather than needing to borrow the money to make the payment. If you haven't spent enough by the time the payment is due, just pay it from savings if you have them, then carry on with with your 0% spending until you've built up a balance to the right amount.
Then you can just pay the minimum on this card to spread the cost of the balloon payment over the rest of the 0% period.
If you do need to borrow to make the payment, The money transfer route might be the cheapest way to do it (I'm not up to date on personal loan rates now that interest rates have gone up, they used to be comparatively cheap, under 3% but I doubt that's the case now).