There are some actions you can do like dismissing followers or transferring them out of the party that will create some debt, even if you have the money. The debt will be repaid the next morning. This was done so the player wouldn't immediately get stuck in a trap of "I can't fire anyone or move them out of the party because I can't afford it right now".
Debt also accumulates if you can't cover followers' daily wages in full at the start of the day, but that doesn't sound like your scenario.