Geoff suggests: Here in the Yew Ess Aye...they yodeled because they hadn't waxed the splintery seat planks on the wagons.

All seriousness aside, I have the WDI* that US'ns cowboy yodeling is imitative of wolves' howls. By suggesting to the cattle that there are predators in the vicinity, they trigger the cows' clumping-for-protection instinct.

*WDI, Whack-a-Doodle Idea