A comment on Wowhead best describes what occurs with the mobs:
Its as simple as this, the quivering filth have a dynamic respawn, any respawn has a chance to be elite. So if you kill them faster, the more likely you are to see an elite one.
To me, this makes the most sense, and I think many mobs in WoW follow this respawn mechanic.
Even if the elite spawning isn't linked with killing the non-elites, it will just seem like they're taking longer to spawn (if you don't kill the non-elites) because there will just be nothing to do.