The vast majority of people don't know how to handle contaminated items correctly, and default to how they normally do things. You can't see contamination like

The vast majority of people don't know how to handle contaminated items correctly, and default to how they normally do things. You can't see contamination like that, so it's difficult to control your subconcious behaviours. Touching the outside of their mask (more likely to have any virus droplets), then touch their face/eyes/etc. Lifting the mask with their contaminated hands to scratch their nose or mouth. Touching the mask and transferring contaminants from your hands onto the inside of the mask, or from the outside of the mask onto your hands then to the inside of the mask. Taking off/disposing of the contaminated masks improperly. Using a mask beyond when it's safe to do so - most masks are not supposed to be used if they become moist, as they can retransmit and help spread droplets.