Film emulsions are affected by pressure. As film is advanced in camera, its path brings it in contact with rollers. These are steel with polished surfaces. Dirt and grit can adhere. When this happens, the surface of the film is lightly scratched and this action acts much like an exposure. In other words, streaks are seen in the developed film. The remedies are to clean the rollers and lightly lubricate them so that they turn freely.
Pressure marks can also result from poor maintenance of the film developing machine. Modern photofinishers often develop film in a roller transport film processor. The cassette is loaded into the machine and the tongue of the film is attached to a “leader card”. The entry door is closed, and the film is pulled over a series of rollers. The path is into the developer tank, then over and out to another and then several more fluid filled tanks. Eventually the film enters a drying chamber.
These marks are pressure marks. The culprit can be the cassette, the camera, or the developing machine. Pressure marks come in two flavors depending on when the pressure is applied and the amount of pressure. Usually, if received before processing they add density to the film. If received during the developing process, the emulsion is disturbed, and this changes the time required for the chemicals of the process to enter and exit. These lines (marks) will be less dense than the surrounds.