this happened to me frequently as well. I have a laptop (with lid closed) running two 32 inch monitors on my desk. Monitor 1 is my left (where I wanted the Fences with my icons) and Monitor 2 is my right.
After troubleshooting some with Stardock, the answer appears to be in the Registry Editor. If you open regedit, then go to Stardock>Settings and then "PrimaryMonitorLast" you will find something like "\\.\DISPLAY1" as a value. Mine would change, for some reason, to DISPLAY2 or DISPLAY5 or something. Anything other than "1" at the end of the string put all the icons/fences on the right monitor (2). So if I edit that to have a 1 at the end, on any reboot, they will still show up on the left monitor (1) as they should be set up to do.
I still don't know what caused this (it hasn't happened in a few weeks) but that's always fixed it for me.