This is one of the never addressed, long-standing bugs with objectdock going back at least 4-5 years. There are many.
The same thing happens at logon/reboot, if your mouse is not hovering over your primary monitor's screen during the startup of objectdock. Yes, the position of the mouse over one monitor or another is the key here.
So if you have docks on Monitor 2, which is to the right of Monitor 1 (your primary), AND you leave the mouse hovering in Monitor 1's screen space, all is well. If, on the other hand, the mouse is hovering over Monitor 2, then the objectdocks that were supposed to be on Monitor 2 will instead appear on Monitor 1.
The only fix is to tell Objectdock to quit/unload and run again. This time, leave the mouse in Monitor 1's screen space until the docks initialize. And all will be well.
As you can tell, I have dealt with this bug every day for many, many years. Fortunately, since Windows 7, I don't restart/logoff my computer often enough for this to be a pain like it used to be. 8P