I was stationed in the Sinai desert for a year back in the late 80s and I remember the discussions that earthquakes had caused some of the port cities to submerge and others to be pushed upward and out of the sea’s reach. All along the northern Sinai there are the remains of Roman outposts between al Kantara (near the Suez Canal) and el Arish (near the Israeli border). During routine patrols in that area it was easy to find the remains of Pelusium - where Cleopatra was unrolled in front of Caesar - and Petrocine with its graving docks and baths. A short stroll unearthed lamps, small statues, coins, and bronze arrowheads. That part of the world is loaded with archeological treasures but the attitude of the Egyptian authorities was “if it isn’t Islamic, we don’t care”.
Also, the city is in the Nile Delta area. The ground was “sinking sand” to begin with, since the river constantly deposits silt, shifts its channel, and generally restructures the topography all the time. Add in earthquakes and storms, and a nice port city can easily be left submerged, like Caesaraea Maritima, or high and dry.
Think of New Orleans and the Mississippi Delta area or the river-mouths area we call Bangladesh. It doesn’t take sea-level changes to make the situation unstable.