There are few things as unnerving as having a semitruck right behind your car, on your bumper, tailgating you as you drive down the highway. Even if you're in a full-sized pickup truck, the semi is far larger and that difference in size is never so apparent as when it's consuming the whole rear-view mirror.
One truck driver wrote about why this happens, despite laws against tailgating, and he said the first reason was simply that other drivers go below the speed limit. Truck drivers get paid for production, so slowing down can cost them money. They want to go the speed limit at all times.
Another reason is when drivers are constantly slowing down and speeding up. The trucker said this often happened because they were distracted. Perhaps the driver of the car was on the phone, for instance, not using the cruise control and not thinking about keeping the speed consistent.
Finally, it sometimes happens when a truck driver is getting ready to pass a slow car. He or she may not be tailgating out of frustration, but simply because it takes a long time for a truck to get up to speed and pass, due to the high weight of the vehicle. With short passing zones, eliminating as much extra space as possible helps the truck get around.
While some of this is understandable from a truck driver's perspective, it is still dangerous and tailgating is still illegal. If a tailgating semi driver causes an accident and you're badly injured, you may need to consider seeking compensation for medical bills, physical rehabilitation, lost wages and more.
Source: About Truck Driving, "TD95: 4 Reasons That Trucker Might Be Tailgating You," Todd McCann, accessed May 12, 2017