By Dan Weil Several factors combined to cause Chinese stocks to slip, as they continued their downturn of more than a year. Chinese stocks trading in the U.S. tumbled Monday, after Chinese stocks listed on the Hong Kong Stock Exchange fell 7.2% earlier in the day. That was the biggest slide in more than 13 years. The drop stemmed from several factors. First, there’s concern that China’s close relationship with Russia could backfire on Chinese companies. Russia has reportedly asked China for military assistance. And U.S. officials have said it will take serious action to make sure this doesn’t …