Visible crew/equipment: At the beginning of the movie, Sheriff Teasle escorts Rambo to the outskirts of town. As the sheriff drives off, the boom mike is visible in the reflection of the car's right rear passenger window. (00:07:30)
Visible crew/equipment: During the motorcycle chase Rambo jumps over the train tracks, if you look close you can easily see the ramp. (00:18:00)
Visible crew/equipment: In the shot of the stolen army truck crashing through the police road block, you can see a camera mounted inside the truck. Look in the small window on the rear wall of the cab. (Apparently both this shot and the shots from inside the cab were done in the same take.) (01:13:10)
Visible crew/equipment: In the scene when Rambo jumps on the hood of the army truck you can easily see the outlines of the cushion they put under the hood to ensure a soft landing.
Visible crew/equipment: When Rambo is jumping down to kill the boar, the camera angle is from ground up, you can see Rambo is wearing some type of white underwear over his jeans, most likely a Jock support underwear. During other scenes the brown tarp he is wearing hangs low so you can't see the white underwear.
Chosen answer: Because it's a knife that someone else is holding and can use against him. It's common for people in dangerous professions to regard their weapons as tools. For example, police officers who commit suicide by handgun rarely use their service weapon, but rather a personal firearm.
Captain Defenestrator
Additionally, I believe it is made clear during that scene that Rambo had previously been tortured at some point, and one of the tools used was a knife. So the combination of being restrained and having someone approach with a knife caused him to basically have extreme PTSD and so he attempts to escape.
oldbaldyone