hit and miss

what are they(truckers) thinking when they change lanes, left to right, just before ascending a hill?  they cut in front of you, slow down due to the hill, and cause everyone behind them to slow down drastically.  and then it takes forever for them to get up the hill and change lanes because they are usually passing another truck.  HUH???? i'm sorry i don't get it.   i don't dislike truckers but this makes no sense at all.