I have used a Low-res monitor as a second monitor before, the problem I had.
Where the resolution differs, there is a virtual wall
This Virtual wall will not allow the mouse to pass, because , there is "no where for it to go" On the monitor which does not have screen there.
THe mouse never crosses that wall, but going really fast the mouse might move more vertically till it pases through the open space.
The location of this virtual wall can be adjusted , by moving the other monitor around in this display setting.
The change in the picture is exaggerated here to show what it is like when the resolutions are way off (which they aren't anymore). This picture shows that where the monitor lines up at is adjustable. You will still have a virtual wall for the mouse to crash into as long as the resolutions are not the same. It is possible in the settings to move where the monitors line up.
If you are racing the mouse across the screen to pass through this virtual wall, it could seem like that is how you got there. I was doing the same thing to try and jump screens with a trackball and getting quite confused too.
Could you test by adjusting the position of the 2nd monitor in the display settings of windows and pass the mouse at the top bottom or middle of the screen WithOut racing the mouse. See if that has anything to do with it?
Realign where these displays butt up against each other in the settings again, and test again. Like align to the top or bottom.
If your in the clear zone where the monitors both meet with the mouse, and speeding the mouse across the screen is the ONLY way to get it to hop over there. Then we need to know what software is being applied to the mouse?
What is the mouse type? What is the mouse Brand? What software is being used beyond the windows generic mouse driver? Other than that there may be settings Hot tracking or x-mouse or something that could be effecting it, although I would not know how.