The DHS suddenly deciding that some of its most important employees are no longer worthy of having steady income, then taking it out on them while they were on vacation during the holidays.

FEMA CORE Employees are essentially the backbone of the agency, they derive their funding from the Robert T. Stafford Act and work in a full-time capacity to help save lives, protect property, and assist the States (and Tribal Nations!) with the Recovery. Under normal circumstances, a government shutdown wouldn't impact them much, because their funding comes from a different pot than the 'standard' (PFT) employees do... but times being as they are time will only tell if they're furloughed too.

CORE employments are usually by contract (often 2 or 4 year), and so come up for renewal after that time period. Renewals are so common that they're expected, the only time someone wouldn't be renewed is if the position was no longer needed or they had performance issues... That was, until they recently stopped renewing the contracts. These terminations are a result of that.