From Peter Whites website-
"If the spoke is long enough, a right angle at the spoke head will cross the line of the hub axle behind the axle, and the result is less lateral bracing of the wheel. That's why I don't build 36 spoke wheels at cross four. With enough side load, the spoke tension can suddenly drop, and the wheel warps. Not a pretty sight."
Almost 1/2 way down the page-
http://www.peterwhitecycles.com/Wheels.asp