Why have Russian Nuclear Engineers and Experts suddenly left Iran's Bushehr Nuclear Plant?

On October 1, the Khorramshar News Agency reported that the entire staff of Russian nuclear engineers and experts employed in building the nuclear reactor at Bushehr suddenly, and without warning, left the facility on September 28 to fly back to Russia.

Could it be that Moscow or Tehran have been tipped-off about an imminent attack by Israel or the US on the plant and so the Russians were taken out of harms way?

Or could it be that Moscow has learned about an imminent Iranian pre-emptive strike against Israel and/or US targets in Iraq and the Persian Gulf region?

At this point these two scenarios are only speculation on my part. But the fact that the entire Russian crew was whisked away so quickly, and without an apparent explanation, from the nuclear facility would strongly suggest that Moscow has assessed a potential danger to its people.

This one bears close watching!