It has long been asserted that processed produce is the riskiest way to purchase it. In this particular case, Taco Bell was receiving lettuce that was pre-shred

It has long been asserted that processed produce is the riskiest way to purchase it. In this particular case, Taco Bell was receiving lettuce that was pre-shredded at a facility. When a consumer or food service kitchen receives pre-processed produce, it's not recommended to wash it again, for reasons. So, the lettuce was last washed before being shredded. Then it was shredded in machines or by hand or something. It may have been misted, I guess, like you see in some grocery stores, but the point is that there was ample opportunity for contamination by equipment, vessels, or utensils, in the processing facility, during or after washing the whole lettuce.