I vaguely remember a situation a couple years ago where one of my credit card balances was increasing by hundreds of dollars everyday. What could cause this?

I was thinking that it was simply the compounding interest rate, but I just asked one of my credit card companies how much interest I would pay on roughly a $700 balance if I only paid the minimum ($25) and waited a month to pay off the rest of the statement balance from last month. They said that I would end up paying about $15 of interest which sounds nearly negligible compared to what I feared would happen. My interest rate is around 18%.

Does anyone know what could cause a situation like the first scenario, where hundreds of dollars were accumulating on a credit card balance every day? (The balance was, at most, $10k, but probably much less.