It isnt always the smartest thing to do when going up a grade, for one semi to pass another, but, in alot of cases, that slower truck on the grade is going to really slow down alot more then the passing truck. He's just trying to get around him, so as to not be bogged down as the climb progresses. He's got 11hrs or more of driving to do each day usually, you only have a few usually. A little patience around them goes a long ways. I can tell you more times then not, the semi driver is the one who gets frustrated at the amatuer drivers that tailgate and cut them off, because they wont use common sense. Saw it plenty of times in two years, nearly killed a few idiots who tried something stupid in front of me. Luckily, for them I had more sense then they did and took action.