A lot of it has to do with remote monitoring systems. Getting struck behind a heavy that constantly has to climb hills at 38mph is incredibly frustrating, especially if their shift points don't match yours because of their load. When a passing lane finally presents itself after 40 minutes of struggling behind an underpowered tractor, the natural reaction is to make the pass. Unfortunately, a lot of rigs have engines governed at 63mph or remote monitoring systems that automatically dock pay when speed violations are detected. If that underpowered load decides to speed up on a straightaway halfway into the pass, an awkward stalemate can ensue, especially when the vehicle being passed is driven by someone with poor driving etiquette.