Maybe I should use the phrase "common factor" - something about these five sites that is similar, but not present for the sites that were not penalized. It could be a technical similarity, backlinks profile, recent global changes to these sites, or many other things.
It's just that you have five examples of this problem showing up at one time, but other sites that do not show the problem. So the obvious question, I think, is what do they have in common that might cause a problem? There good qualities are not the cause of the ranking problems, you know?