Sayeth the commentators:
"There's not a sound football guy in the world [who would have kicked the onside kick when Leach elected to]"
Leach doesn't think like other college football coaches. That is, in part, why he is so effective (I'm attributing the rest to his emphasis on the fundamentals *of his scheme*). Therefore the rationale for the onside kick - an unexpected onside kick is much more likely to be effective than an expected onside kick. By taking the chance he looked to put his O back on the field and continue to make the Longhorns suffer from his attritional style, whilst at the same time relying on his D who had played excellently in the first half.
A sound football guy should know that the unexpected is effective - see PLAY ACTION PASS and DRAW for more details. Of course, it isn't always (Seahawks 3rd down draw scheme) but that isn't to say that we should criticise coaches for taking shots and risks.