One guess might be that winters in Europe were harsher than other population centres (China, India, Middle East), and periodic famines meant that some people would just scrape enough calories from milk. You hear stories about people trying everything when they're out of food, maybe trying the cow's milk was a sort of last resort that worked for some lucky people. Then again, you'd think they'd have eaten the cow first.
The article does say it happened too fast to be from natural drift or population turnover, but I'm not sure what turnover means. Is that replacement say from a conquering tribe?