All frivolity and sarcasm aside for a minute, DHS has had an internal policy for years recruiting volunteer "surge" support for their various administrations and agencies. (This is most common for FEMA augmentation, which isn't limited to just DHS employees. During periods of significant disasters and clean-up, FEMA has solicited volunteers from other departments.) Employees from other parts of DHS sign up to volunteer to be deployed to support other DHS components and are placed on a list based on their skills and physical fitness for field jobs. They earn overtime, hazard pay and shift differentials, etc, according to DHS and OPM rules. Volunteers are supposed to get their supervisor's approval before being deployed. DHS simply can't get the job done with internal ICE and CBP resources, so they have dug into the TSA volunteer pool. Given the extreme visibility of the border crisis, I'm sure DHS dispensed with the supervisor approval part.

The Bloomberg article Petaluma1 linked goes into a bit more detail.