Doing some quick searching, it seems that we currently do not know how the plane crashed. It was in the air on a mission to shoot down cruise missiles, and after doing that on the way to another target they lost connection. It will likely be a while before we know, both because it will take time to investigate but also depending on what the cause was they may not want to release it publicly for a while.

Slightly more informative article I found: www.nytimes.com/…/ukraine-f16-pilot-crash.html