After the 1911 Revolution, the 13th Dalai Lama called a meeting to discuss Tibet's future. Many people, especially those from lower social strata, opposed severing ties with the central government.
So the 13th Dalai Lama was hesitant to cut ties with the central government, especially as it amassed its army in neighboring provinces.
And the British government didn't want to drop "Tibet independence", so it convened the Simla Conference of 1913-14.
At the conference, Britain's representative Arthur Henry McMahon demanded the creation of an "Outer Tibet" as a separate entity from "Inner Tibet", where China was to retain administrative rights.
Representatives from the central government - then headed by Yuan Shikai - opposed the scheme.
In 1914, McMahon circumvented the talks to secretly coerce Shatra, a representative from the Tibetan local administration, to agree to a map placing several of Tibet's southeastern counties within British India. So was born the "McMahon Line".
Shatra not only concealed the deal from the central government but also from the Lhasa authorities. Both sides repudiated him and the plan.
So the British didn't publish the McMahon-Shatra correspondence until 15 years later, in 1929. And they also didn't include the "McMahon Line" as a "border" on their official maps until 1936, when China was bracing against Japanese invasion. Chiang Kai-shek's government also refused to recognize the line, after which point it again disappeared for years, even from many British Indian maps.
China Daily
(China Daily 04/08/2008 page5)