Unclean hands can contaminate fresh produce with human pathogens such as hepatitis A. Workers can walk through contaminated dirt and climb up ladders placing their hands on rungs they have just soiled. Often, workers sit on their picking bags while taking a break. Baskets are placed on the ground and can easily get contaminated soil in them. Thus, previously pristine produce can be subsequently contaminated during harvest. The processing facility itself can be left open to contamination from dust or animals.
Dirt floors can contribute to the risk of contamination.
In the processing shed, produce can be rinsed with unclean water, whether contaminated by dust or animals or previous produce passing through the process. Tools such as filters, knives or fingernails may not be sterilized between processes or may left in a place where they can again become contaminated via dust, nonpotable water or animals. A part may fall on the floor, be picked up and put back into the process again without being properly cleaned. Lastly, the final packaging materials, cartons, etc. can be contaminated by previous fruit or tainted water, by being set on contaminated ground tainted by manure, or by being transported in a vehicle that previously held animals. As long as they weren't grown in raw manure, you might think sprouts would be a relatively safe form of produce.