For a lot of them they do like the challenge, it's what drives many agency analysts/operators, but for many it was the lure of money. Packages up to half a million depending on your former position/credentials, free housing and more were throw about like candy. Some of those leaving the agencies were former military or newly hired GS9's. You can see how the monetary lure would pull you out.
A bigger problem that has persisted is why are they leaving the domestic agencies? What needs to change for retention there?