Though CT has usually been strongest among the far right -- where everything was a Jewish or Communist plot--there has been an overlap between leftist politics and CT for a long time.
Back in the day, people who were fighting for good progressive issues--civil rights, economic fairness, stopping the Vietnam War--sometimes fell prey to dualistic thinking and willful blindness and started thinking the USSR/Russia must be OK since it opposed the USA. Russia is the place that invented the granddaddy of all conspiracy theory hoaxes, the Elders of the Protocols of Zion. The "globalist agenda" from the OP, used by both the right and the left, is just code for The Jews Run the World.
This kind of overlap continues to this day, with people on DU pushing RT videos and pro-Putin propaganda mixed with conspiracy theories--and it's even more bizarre now since Russia no longer pretends to be the land of Soviet equality but is now a hyper-capitalist dictatorship overtly shutting down human rights. RT (Russia Today, funded & run by the Kremlin) recently got caught with an "expert" host who was in fact the editor of a German neo-Nazi magazine, so nothing much has changed.
RT promotes Birthers, Truthers, climate change deniers and even the evils of water fluoridation, while giving airtime to pro-militia "patriots" & dimestore racists. Same for "New World Order", the Illuminati, Bilderberg conspiracy theory, all stand-ins for Jewish world control anti-Semiticism. The anti-Semiticism CT even envelopes Hillary Clinton in the RT bizarroworld.
Have you ever noticed that another major mainstay of conspiracy theories, that the moon landings were fake, just happens to involve the fact that the US is the only country to have landed people on the moon?
There are other reasons for CTers on a progressive website like DU, and some might have other influences than the historical connection between CTs & anti-US extremism. For example, environmentalism overlaps with chemtrail CTs. But it's hard to get away from that historical overlap between anti-US politics and CT.
Some links
https://www.washingtonpost.com/news/monkey-cage/wp/2014/08/21/conspiracy-theories-arent-just-for-conservatives/
http://www.theguardian.com/commentisfree/2015/dec/18/conspiracy-theories-extreme-right-far-left-threaten-democracy
https://www.splcenter.org/fighting-hate/intelligence-report/2010/russian-tv-channel-pushes-patriot-conspiracy-theories
http://rationalwiki.org/wiki/RT#Conspiracy_theories
http://rationalwiki.org/wiki/List_of_conspiracy_theories
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