i think this kind of precessing is being more frequent these days. a friend of mine who is doing PhD in biophysics (also in ranked school) recently went through the same situation. THough he is back in US already, he went to nepal for a usual vacation and was put on hold while applying for the visa. I know him well as a smart kid and has a good undergrad gpa from a good school. He himself does not know why they put him on hold rather than told that was just for processing and checking. really frustrating.