Many of these aircraft have been in "active storage" with some fuel remaining in the tanks. Although that fuel is often treated with biocide, the threat of microbial contamination still exists. This is because fuel is warm for extended periods without being in flight and fuel is also static, so “hotspots” of contamination may occur that are very difficult to detect.
Read more:https://www.aviationpros.com/gse/fueling-equipment-accessories/quality-control/article/21149610/how-fuel-contamination-threatens-grounded-aircraft