Well, speaking as a humanities major let me say, huh?
I assume it's simply physics, and the amount of force in one direction is attentuated by that force not being entirely from that direction.
Here's an online calculator:
http://www.aeroplanner.com/calculators/avcalcdrift.cfm
And here's one similar to the one I used to use in piloting my little Cessna and later Cherokee:
http://sportys.com/acb/showdetl.cfm?...oduct_ID=10238
If your question is about how is a particular aircraft's maximum crosswind component determined, I believe it is a matter of both wind tunnel testing and test flying to the point at which the pilot needs Depends.
Iirc from my now-lapsed pilot days, low-wing planes generally have a higher crosswind component than high wing ones (e.g., most Cessnas and most Pipers, respectively). The highest I ever landed in was a 90 degree 15 knot crosswind in a Piper Cherokee; I believe that was almost at the maximum. It was a puckery ride but the airport had only a north-south runway.
LarryJ can probably give you a better answer; maybe he'll come calling on this thread.