Thanks for the reply. I knew you would add something interesting.
I think it was a massive drought.
The Hakka (Chinese), who I think are the Asian equivalent of the (European) Picts, migrated all the way from the Tarim Basin region to Southern China. To this day, they are known as 'the guests' in China.
I think the people from that region were of mixed Caucasian/Chinese race and the mainly Chinese mix (Hakka) fled into China and the mainly Caucasian mix (Picts) fled west.
The Hans later conquered the whole region to the Caspain Sea and even made contact with the early Roman empire.