Many Egyptians have believed for 30 years that Israel chose to implant its embassy in this specific location in order to be able to fly its Star of David, blue and white flag over the Nile.
And indeed that is what they think. This Facebook group talks about it, and the Egyptian who took down the Israeli flag from the embassy in August claims that he saw a banner on top of the building that said, in Hebrew and Arabic, "The Land of Israel from the Nile to the Euphrates."
Even Al Ahram notes that most Egyptians are so irrational as to believe that Israel would choose a site for its embassy, not for security, but to engage in a symbolic annexation of a good portion of Egypt.
Ma'ariv reports that it took US threats to withdraw all aid to the Egyptian military and take away its own embassy personnel before the government decided to help out the Israeli guards who were facing a lynch mob. It also reports that Israel is looking for a new location for the embassy - presumably not on the Nile.
(h/t Yoel)