The Mayor of Ponyville is millions of dollars in debt - not from repairing Ponyville after the parasprite infestation, but from gambling. She spends weeks at a time trying to win it all back in the casinos of Las Pegasus but always ends up losing everything. And then some. In poker. To Derpy. Who to this day just doesn't know what went right.
Desperate, she sought the help of some loan sharks. Yes - these are actual sharks that give out high-risk loans. They're the ones who keep causing all these disasters in Ponyville to show the Mayor that they mean business. Kids, don't trust a loan shark - they will come back to kill you when you don't pay up.