[@BCTheEntity]
[hider=It's magic!]
[img]http://i1.kym-cdn.com/entries/icons/original/000/018/816/its-magic-i-aint-gotta-explain-shit2.jpg[/img]
[/hider]

No, but seriously, it's literal magic... all the available psykers would be too busy looking out for eldar mind controlling key battlefield personnel, to be explicitly scanning for someone tossing off small-time powers.
And by small, I mean compared to the fluctuating void shields, vortex missiles, and general eldar presence happening all around it. He'd be just another drop in the bucket for things fucking with the localized area of the warp.
So, literally, magic... "I'll do something over here, but you won't see it because you're looking over there."
Here being warp plasma, there being a vortex missile.