What defines the plane of an accretion disk around a black hole?
Let me give an answer based on my intuitive understanding of the thing...

 Initial conditions

The initial accretion disk movement is defined by the objects falling into it, the black hole's mass and its spin of course.

If you take the situation where a single star falls on a quiet black-hole, the accretion will appear along the orbit of the infalling object.


Credit: NASA, S. Gezari (The Johns Hopkins University, Baltimore, Md.), A. Rest (Space Telescope Science Institute, Baltimore, Md.), and R. Chornock (Harvard-Smithsonian Center for Astrophysics, Cambridge, Ma.) and live on youtube...

 Equilibrium in an active galactic nucleus

Now on the long run, if additional matter is constantly coming in, at first thought I would think 


the accretion disk momentum will be exchanged with the one of the black hole through the Penrose process, 
Infalling matter momentum will reflect the overall host galaxy momentum - especially if the galaxy is a spiral