Hi Lan Brown,
In SAP Personnel Time Management many options available to cover the client requirement, now come to your queries where you have asked tow things;
1) How can the absence quota start negative? 2) What causes that to happen?
Here i believe that you looking for real business scenario that how is it possible or in which scenario absence quota start with negative, answer is yes that if business scenario is an employee wants leave and absence quota does not available due to prorate calculations then it goes in negative.
Here in SAP Time management there is back end settings that system allow that employee avail his leave even he does not have enough absence quota, then this negative figure auto adjusted once absence quota generated in upcoming month/next month.
Hope you got some idea,
Regards;
Bashir