Apple - Why does my dock keep moving back to my other monitor?

You can summon the Dock on a different display by moving the cursor to the bottom of the desired display, and then continuing moving down. It may be possible that this is occurring when you inadvertently perform that action.

I answered a similar question: cmd-tab behavior on Mavericks with multiple displays.

If you can't drag the mouse down due to stacked monitors, or due to some other resolution differences between monitors. Go to System Preferences → Dock → Position on screen → click Left, and the dock will move back to the primary monitor.

Then you can go to the Bottom screen and follow the steps to summon the Dock, and it will return there.

Apple didn't account for the over/under monitor configuration, the overhang catches your cursor and summons the dock. In this case, the switching is constant and annoying.

One workaround is to put the dock on the side of the monitor, which will prevent the summon when the overlap catches your cursor.