Why does Mac OS X turn off monitor #2 when an application is maximized on monitor #1? I am running OS X Yosemite 10.10.5 on a Mac Mini system with two monitors. When I maximize an application on monitor 'A', monitor 'B' then goes black while that application maximizes to fill 'A'.

Why the heck is this happening? How do I fix it?

Thanks!
Why?
Personally,. because I don't think they ever thought it through properly. There's a lot about fullscreen that's like that. 

The fix is possibly System Prefs > Mission Control > check Displays have separate Spaces



The better fix is don't use fullscreen, use Alt/click on the green button to maximise instead.