Well, I was thinking of maybe having some Colony leader in the edge of Alliance Space be secretly Indoctrinated in the past. He is left with a special command for him to start rounding up people at a certain time (Which is NOW). It's all set to flawlessly line up with a certain shipment of Dragon's Teeth and an Indoctrination Altar to teach the "doctors" some new techniques...
You see, with the death of the Collectors, the Reapers need new slaves. They just need to make them, first.