Dock randomly switches between screens. Any way to stop? [duplicate]
Dock position could be changed when You place Your mouse on the bottom edge of the secondary screen and then move it down again.
I know this is old but I came across this and thought I would share my solution.
My fix is to disable separate spaces by going to "Preferences > Mission Control" then disable "Displays have separate Spaces.
The only downside to this is that you will now loose the second menu bar as well. Thankfully there is also a app for that if this is something you want. Its called "SecondBar" and can be downloaded from here. http://blog.boastr.net/