http://www.sci-news.com/archaeology/peopling-tibet-chusang-04514.html
“Among some of the best preserved sites for scientists to study is the Chusang site, which is located on the central plateau 50 miles (80 km) northwest of Lhasa at an elevation of 14,000 feet (4,270 m), near Chusang, a village known for its hydrothermal springs and extensive travertine formations.”
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Hydrothermal springs might provide a clue...that valley may well have been ice free.
A last refuge for the survivors to fall back to as the ice piled up around them, is my guess.