These two images show what happens when you click over from -609 to -608.
I discovered it by accident, and was quite surprised. Nothing I could find about those years indicated any kind of large-scale public works project or ecological event that could have caused the entire gulf to recede so far.
Did something like this really happen? What might have been the cause?
I cannot find any evidence of a major shift in landmass in this period. The more likely explanation is that the author decided to process the recession of the Gulf in one go. One has to remember that in Ur's ancient history, it was near the mouth of the Euphrates. Looking at its location in the modern day, it would be around 150 km from the Gulf. Furthermore, the borders of the Gulf that are used in the year 608 BC are those resembling the modern ones. Ergo, the most logical explanation is simply that the author processed the increase in the floodplain/delta/marshes of the Euphrates/Tigris river system between ancient history and the modern day in one go - between 609 and 608 BC.
I don't know if this is too off topic, but how reliable/accurate are that site's maps
Look how smooth that extension of the Gulf is. I'm suspicious that it's just an example of someone fat fingering drawing something.