Vanguard takes screenshots of your PC every time you play a game. Every time you play a game a function is called to screenshot your PC's screen, in case Vanguard thinks you might have something suspicious, it screenshots your ENTIRE PC screen (all monitors).
Or MAYBE, people who enjoy "hacking" (the actual programmers, not "cheaters") and go decompile/learn the inner workings of programs might have a better idea of what's going on? Just a wild guess.