The culprit is more than likely static. Very simply, static electricity occurs whenever two materials are mechanically worked together, or when two materials in contact are separated rapidly. Through a process called tribo-electrification, electrical equilibrium is disrupted. One material loses electrons to become positively charged while the other gains them to become negatively charged. The material's conductivity determines its ability to pass along this charge. Good conductors, such as metals, can lose a static charge almost immediately. Insulators, such as plastics, retain their static charge - sometimes for weeks.
Many normal handling operations generate static. These include parts moving on conveyor belts, sliding down chutes, or simply rubbing against each other. In fact, manual cleaning with tack rags or using a compressed air blow-off to remove visible dirt will impart an invisible static charge to attract and hold more dirt as the part travels through the production pipeline.
I even had serious issues with our UV flatbed. The winter is the worst time.