The only reason for a negative account balance is a survey chargeback. In order to check if you received a survey chargeback, go to your surveys history and look for an entry with the type Chargeback.
A chargeback happens when a survey that you completed in the past and that you have already been credited for is being invalidated. Chargebacks can't be reserved, they are created by the survey provider.
Please read this article to learn more about what chargebacks are and how they work.