Recently, I picked up my truck from the shop with several mechanical issues. DOA on arrival. Would not start. After Diagnosing the problem, the technician discovered a faulty fuel control module and the #2 injector failed. In an unrelated problem, I was also experiencing a variable pitch whistle coming from the turbo. The mechanic discovered (after he tore apart the the turbo) rust inside the housing. The vanes were also sticking as a result. Here is where my question lies.

What exactly causes rust to form inside the housing. I've read in other posts that if the truck sits for extended periods of time, this can cause this. Is there any other circumstances when this might occur? The mechanic completely cleaned away the rust and and replaced the unison ring and replaced the center housing. He also cleaned the exhaust side and replaced the turbo drain tube. After roadtesting the recheck produced 25psi boost.

Any thoughts.

Thanks