this is an honest question, and I am only curious because it seems like every event I read about if you read just a little to far into it. at some point, some one brings up the fucking reptilians. so I am interested if any one knows when this particular subject became popular, or how far back does its origins go.
Conspiracy theories are a big guilty pleasure for me, especially about reptilians, I have no idea why, but here's an attempt to answer your question:
It's difficult to talk about the history of conspiracy theories since most of the theorizing occurs on a very unorganized, individual basis. The reptilian or alien conspiracy became popular in the 1970s, as an outgrowth of the more general New World Order/Illuminati/Freemanson conspiracy theories. David Icke in the UK and Milton William Cooper in the US were both reptilian/alien conspiracy theorists.
The larger "New World Order" conspiracy has its roots in late 19th century counterculture, especially religious fundamentalists with apocalyptic millenarian beliefs that grew out of the Second Great Awakening (Shakers, Millerites, etc.) in the US. The popular esoteric movements in the early 20th century (like Theosophy) also popularized the idea of a "secret knowledge" and an overarching "global power". The term "new world order" was created in earnest during the early 20th century, as the shifts in global power and politics after the world wars led to the idea that a "new world" of global peace and democracy was really beginning. H.G. Wells (I think) first appropriated the term to refer to a dystopian global state. During the "Red Scare", amid fear and paranoia about communist takeover, William Guy Carr became a popular conspiracy theorist using ideas that the Illuminati/Jews were behind communism. The xenophobia and antisemitism of the Red Scare permeated his theories.
The reptilian/alien theories are often accused of being thinly-veiled metaphors for Jews or foreigners. Icke has denied this, but it's pretty easy to see how the reptilian conspiracy displays a foreign, outside, anti-Christian force taking control of the lives of ordinary, hard-working people, which is basically the narrative of non-conspiracy-theory right-wing anti-immigration politicians everywhere.
TL;DR: fear and paranoia in the 20th century + religious fundamentalism + fear of government intrusion into life + offensive Jewish caricatures = reptilian agenda.
Related question: can the legend of "reptilians" be in some way a modern retelling of the Fair'Folk/Changeling/Fae Myth, the same way the "greys" are suggested to be?