Salsas are pretty acidic from the tomatoes and lime juice so they're less likely to be the culprit than mayonnaise, the most vilified condiment (not only does it clog your arterties but it's a safe harbor for bacteria :shocked2I suspect it is related to the salsas left on the table and the ability of E. Coli to grow, or the high ambient heat overcoming the ability of refrigeration to keep food stuffs in the ideal temperature range to inhibit bacterial growth.

Tomatoes have a pH of 4.2-4.3, while ETEC can only grow in a minimum pH of 4.4. Most bacteria prefer neutral pH ranges.
Chapter 3. Factors that Influence Microbial Growth