I have had a number of sites removed from the Google index despite having done nothing out of the ordinary or in violation of their guidelines. Despite repeated emails and support requests these sites never returned to status they had before.
Why did this happen? I honestly dont know for sure. Though I obviously dont have concrete proof. I have always believed it was sabotage by a competitor who called Google and established a contact there and used (uses) that relationship as a way of damaging other webmasters who are/were having success in search terms this person wanted.
Why do I say this? I had a chat once with an affiliate who literally told me their “inside contact” at Google told them that two highly ranked sites were going to be “banned” from the SERPS. Sure enough within a week one of the sites vanished from a top 10 position for the term “Online Casino” and during that same time frame one of my sites which had been in top 5 for the term “Online Casino” also disapeared from the serps. Both sites had been successful in Google for years, featured unique content and did not employ any tricks that I know of (I know mine didnt anyway) . Neither my site or the other affiliates website ever came back in the serps for even minor terms though PR and back links remained steady. Hence my belief that sabotage does occur at google and once the damage has been done its seemingly impossible to reverse.