Just ran into this issue today on a large build (11GB). (2017.4.6f1) All of the processes in the task manager became windows and anything in my main screen became unselectable until I could kill explorer.exe. The issue happens right after the build is opening the folder location. It happened 3 times in a row, all after a fresh PC restart with no other applications open. I did a small build in the same scene (Only a few scenes checked in the build setting) and it built just fine. After that I enabled all scenes again and did a full build and the issue has gone away. fingers crossed it stays like that. I forgot to screen grab the error codes that were in the editor. Something about windows. My guess is that Unity had a hiccup and instead of pulling the windows explorer forward it just pulled all processes forward.