Why does moving my mouse pointer towards the Dock often move it to other monitor?

I'm not talking about summoning the Dock--I know how that works--what I'm describing is essentially the opposite. I don't know why it happens, but it happens about 20% of the time I move my pointer towards to Dock, the Dock will just move to other monitor. It's annoying enough I can't keep the Dock on one screen and still have spaces, but this just defies logic. I don't know whats causing it, if there is an obscure gesture I am accidentally doing which triggers it or if its just a glitch. It happens on both my MacBook Pro with external monitor and an iMac with external monitor.