Most likely the AVC (Active Vibration Compensation) dropped out - I hope. This gives a very marked increase in felt vibration, and happens from time to time although is not serious at all if that is what it was. There are plenty of other non-serious causes that are possible, and the crew did the right thing to return as they were so close to a deck, as there would have been some doubt and any increase in vibration should be investigated. This has happened plenty of times before and has almost always been a non-serious cause that could have been safely flown away indefinitely. Don't automatically assume the worst.
It is a bit unfortunate though that every single action is now publicly published and scrutinised before any cause or information relating to the incident is known causing increased panic and paranoia where it is most likely not justified. If it is a serious incident, then by all means discuss and print, but why not wait until more details are known. This does not benefit anyone in my opinion.