What are two possible ways that the potato salad became contaminated with the bacteria?

In most “salads” of this type, low-acid potatoes, chicken, pasta or hard-boiled eggs are added to the mayonnaise. The mayonnaise is acidified to make it safe, but the low acidity of the potatoes (or foods) offsets the acidity of the mayonnaise, creating an environment where bacteria can thrive

anxious,the clues in the question suggest he was embarrassed and anxious about boo radley.