Well, we got the idea from the SAU-Vic DECA days.
Part 1: everyone knows we commonly refer to the Victorians as "Mexicans," and we're North of them, as are Texas, California, New Mexico and Arizona in the States. Seeing as Texas is the most often related to when discussing sharing food, culture, etc. with Mexico, as well as the majority of the border in comparison to the other US states, it was easiest to use this as a reference to us in NSW.
Part 2: the event we hold is based on motorkhana-style runs on a skid-pan, with a hill-climb thrown in for good measure.
Add parts 1 and 2: Texas + Motorkhana = Texi-Khana.