Why would my ex's attorney want my personnel files for a child support case.

Whenever the Court addresses child support, it must determine each parties' income. In the event either parent has left employment by reason of being fired or quitting then the Court can decide to "impute" income to the fired/ quitting parent at the rate they were earning, especially if it is higher than the present rate of income. To prove that one was fired or quit, the opposing parent subpoenas the employee file of the other parent.