The World Health Organisation s International Agency for Research on Cancer (IARC) exists to call things a carcinogen. In their entire history, they have failed to find something carcinogenic in their 900 efforts one time. The reason for their success is simple: In a mainstream media world, combined with the ability to detect almost anything anywhere, it is simple to gather a group of people predisposed to a conclusion, meet in secret, give scientists who might disagree little time to respond, and then release media talking points as fact.
Given the nature of IARC, calling glyphosate probably carcinogenic is actually something of a victory for the evidence-based side. It is assumed IARC would declare every pesticide not used by organic food farmers to be causing cancer, even though they lacked any evidence. IARC does not do any new studies, they simply "review" other work. Regardless, they reached a conclusion in total that none of the studies did individually.
The American Council on Science and Health produced a peer-reviewed analysis in 2011 that was conclusive on the issue, and the EPA has agreed after numerous re-registrations.
WHO Says Monsanto Roundup Ingredient Is 'Probably Carcinogenic.' Are They Right? by Alice G. Walton, Forbes