I'll cite two articles - one is Daily Kos, so take that with a grain of salt.
Initial reports believed that the crater was caused by a methane explosion as a result of permafrost melt. The first team on the ground says that there appears to be no evidence of explosive forces, and rather, this may have been caused by sheer pressure. There are many oddly-shaped lakes in the region that were formed many thousands of years ago; some believe that this process may be how they were initially formed.
Source 1Source 2
