The reason for this was a sharp climate cooling that occurred about 5.5 thousand years ago. At that time, the forest thrived at altitudes that were higher than the current coordinates of its growth. The journal PNAS wrote about it.
This suggests a climate change that occurred after the end of the last ice age about 10 thousand years ago and the moment of death of these trees.
The high altitude forest was an active ecosystem with animals and ancient people hunting. They also found a 10,000 year old wooden pole. It was presumably part of a hunting spear.
The trees probably died because of the gradual cooling of the climate, which ended the warm period. Then a series of volcanic eruptions caused ash and other materials to be released into the atmosphere, triggering further cooling.
This volcanic cooling was so intense that ice quickly enveloped the trees, preserving them until the present.
While under the protection of the ice, the trees have retained their shape, though they have lost their bark, and look like modern trees from areas with high winds.
The glacier in which they were found never melted, providing the trees with protection from destructive processes.
It was also reported that in Canada found pterosaur fossils with traces of crocodile teeth.
By the way, archaeologists found in the burial ground Shamanka-II on Baikal another remains.