Rashi (18): Yirmeyah (43:13) prophesized about it in Tachpanches - "he broke the Matzevos in Beis Shemesh in Egypt." It says Etzel Gevulah because it was on the border of Egypt. Malbim - it is near the border, for a sign that Hashem is Elokim, and He will wage their battle against one who comes to their border to harm them.