Madok Erl suggests: [source: "primus deamon" vol14; chapter 17 pp723-725.]
That a handful of new born half-daemons, who had not yet received the implant gene, escaped this mass cull. The new hybrids were en-route from earth to the homeworld, when the termination policy was announced. Immediately the daemons rebelled against their alien captors, forcing the ship down back to earth and making their escape from their prison vessel.
The hybrids managed to find their way to Hobbs,(or Hobbs found their way to him), who offered them sanctuary against their Alien creators.
An Alien hunter team were dispatched to track down and exterminate the last of this hybrid breed and soon arrived before Hobbs - A tense standoff ensued, the Aliens promising war, and Hobbs welcoming it.