That is a shocker. I had to check Kunming's location, it is near the Vietnamese border and a very long way from the known, occasional flash point in Xinjiang Province - where knives have been the preferred weapon in attacks. This maybe a repeat of the jeep attack in Tienanmen Square last year.
I guess it could be tied to the Uighers, but I think there are quite a few other possibilities. Heck, it could be a Green Peace like movement on steroids. If you Google unrest in Kunming you'll find several articles going back a few years.

http://www.theguardian.com/world/201...ce-baha-attack

Chinese villagers attack factory after reports of polluting

Environmental protests are on the rise in China, with the public becoming increasingly critical of the fouling of the country's air, soil and waterways during decades of breakneck development. The unrest poses a serious political challenge to the Communist party – anger over the party's response, or lack thereof, to environmental crises has fuelled wider dissatisfaction with corruption and a lack of official accountability.

Most protests have taken place along China's developed coastal region, reflecting the area's heavy pollution from industry as well as the rising demands of the country's well-off. But the latest unrest was in rural Yunnan, indicating the protest has now spread further inland.

Yunnan's provincial capital, Kunming, was the site of large protests last year against a planned petroleum refinery that were largely peaceful despite minor scuffles between demonstrators and police.
Maybe more to the point, is the rail road itself?

http://www.japantimes.co.jp/life/201.../#.UxJds42Ybug

Railroading debt into Laos

The construction of an ambitious rail link between Kunming and Vientiane reflects China's growing economic clout in Southeast Asia

However, the plan sparked surprisingly vocal protests from Lao villagers whose rural communities would be at threat of being bulldozed away.

Now, both governments involved have become more determined in their bid to push ahead, and one-party states are not known for being listening for too long to the voices of those opposing bureaucratically ordained projects.
Then again terrorist attacks are not unheard of in Kunming, in the run up to the Olympics in 2008....

http://ww4report.com/node/5818

China: Kunming blasts signal growing unrest in countdown to Olympics

BEIJING — Two public buses exploded during the Monday morning rush hour in the city of Kunming, killing at least two people and injuring 14 others in what the authorities described as deliberate attacks as China is tightening security nationwide and warning of possible terrorist threats in advance of next month's Olympic Games.
While it is difficult to cease speculating, I don't claim to have a clue on what actually prompted this attack.