According to Stichen, the Annunake planet comes around every 20K years. He says that we were made for slaves but they we had became too numerous and noisy and the flood was supposed to wipe us out - except for one god that transported selected humans for safety.
Because there are half-breeds as (angels mentioned in the bible), that are not fully human and have a more of a reptile personality without emotions.
The experiement had went horribly wrong and after the first flood - other Annunaki said since 1/2 Annunaki/humans are living that those shouldn't be destroyed outright.
Perhaps the Second Coming is when they come back and eat the grain-fed pig/cow/human flesh round up for thier planet.
Those left behind on Earth would most likely be the the stringy, tough humans for the next cycle of breeding.