Revealing mistake: When Rambo throws a rock at the police helicopter and knocks Galt out, you can clearly tell it's a stuntman. Galt is in his 40s and overweight. The guy that was falling looked like he was 25 and was noticeably thinner.
Revealing mistake: In the scene where Rambo is driving the truck and pushes the cop car into path of the broken down car, the explosion starts the second the cars make contact. In the next shot during the fireball, the two cables that held the static vehicle in place are visible.
Revealing mistake: Towards the end of the film where Rambo shoots up the computers in a building and the spent rounds are falling to the ground. It is clear to see that these are blank rounds. Where the bullet would sit in the case normally you can see the case is extended, This is where on a blank it is just crimped closed due to the lack of a bullet.
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