Let's walk through the evolution of a mold claim due to roofing repairs or interior restorations.
The building owner calls and reports the leak or damage. Implied in this request is water damage and the intrusion of outdoor elements, like mold, mildew or other various spores.
The roofer assesses damages and fixes the roof. Perhaps other contractors are called in to repair sheetrock walls or ceilings. Everything looks back to normal if not better, and the roof does not leak during the next storm event.
A few weeks later, visible black mold is present in the repaired area. What went wrong and why is the contractor to blame?
Part of the repair process is drying out the dampened area. Mold needs over 15% moisture content to thrive. That level feels very dry to the touch; dry soil is about 17% normally. Wood members need to be that dry because wood is mold food, and it is very difficult if not impossible to remove a colony of mold from a porous surface.
Use a moisture meter to measure the dryness of the repair area before covering up wood with insulation or plastic. Do not encapsulate wood greater than 15% moisture content. Treat any wood exposed to the elements with a biocide before covering it up. This includes new building materials.
Advise the building owner to positively pressure the building to avoid mold infiltration. In other words, circulate filtered air and let the building exhale normally.
If you use these preventative actions, environmental claims will be reduced. Look into Contractors Pollution Liability to cover these occurrences.
To answer the opening question: it is not the roofer's fault. All the conditions existed for mold growth before they arrived on the scene. However, as a completed operation, roof repairs assume the wood members and other roof materials have been returned to fully functioning condition. Wet wood does not meet this definition because rot and mold is likely to grow on it.
Dry thoroughly - and test the moisture content to verify dryness.