Explaining the prevalence of Sikhs in the trucking industry definitely requires one to risk flirting with the 20-year rule. As a very general answer, trucking is a high paying job that doesn't require an extensive education, is easily accessible to recent immigrants who may not have access to more complex job training, and allows Sikhs to practice the outward aspects of their faith without much risk of discrimination (it's easy to wear a turban while driving a freightliner, not so much when building a skyscraper or, in the past, in most whitecollar jobs).

This being said, Sikh presence in blue-collar resource extraction industries on the North American West Coast has a history dating back to the early 20th century. This is a topic I have written about in the past. I have reproduced an answer I wrote earlier regarding why there is a large Sikh diaspora on the North American West Coast, it will hopefully shed some light on why Sikhs drive trucks. The most immediately explanation is that, compared to other immigrant groups, Sikhs arrived on the West Coast relatively early and established local diaspora organizations that would aid later immigrants in their arrival.