Yup its the organic compounds, many will have an overall neutral charge so will remain in de-ionised water. This effectively creates a more nutrient rich mixture, compared to distilled, that would allow microbial growth an easier foothold in what tends to be a comfortable environment temp wise.
Although you could run deionsied with anti-microbial additives, going DI is a nice further (and simple) safeguard in my opinion.
You could even go so far as tubing colour and not using a clear sidepanel as extra steps, if you were paranoid. *eyes darting left and right*