See also here:
http://news.nationalgeographic.com/news/2014/02/140204-dinosaur-fossil-china-volcano-pompeii/
Ancient Ash Volcanoes Entombed Chinese Dinosaurs
After analyzing fossils and sediment, Baoyu Jiang of China’s Nanjing University and his team concluded that lethal, sudden pyroclastic volcanic eruptions marked by air blasts, hot gas, and ground-hogging clouds of fine ash likely smothered, charred, and then carried forward everything in their path to create these bone beds, according to the study published in Nature Communications.
The finding explains why so many creatures would come to be buried on lake floors, and how they remained well preserved enough to retain signs of soft tissue features, such as feathers, tens of millions of years later.
Another possibility is that the animal bodies floated into the lake or were washed there by flooding. The researchers ruled these scenarios out, because the structure of the sediments and the intactness of the animal skeletons didn’t fit with these explanations.