Another interpretation: they said, now they are coming to stir up the quarrel that was between our father and their father, as it is said, "And Esau bore a grudge" (Genesis 27:41), and so forth. "Trembling seizes the mighty of Moab" (Exodus 15:15). If you should say that they were coming to inherit their land — has it not already been said, "And the LORD said to me, do not harass Moab" (Deuteronomy 2:9), and it says, "and you shall draw near opposite the children of Ammon" (Deuteronomy 2:19)? Then what does Scripture teach by saying "Trembling seizes the mighty of Moab"? On account of grief.
Moab Trembles Lest the Old Feud Be Reawakened
Yalkut Shimoni on Torah 251:12
דָּבָר אַחֵר אָמְרוּ, עַכְשָׁיו הֵן בָּאִין לְעוֹרֵר מְרִיבָה שֶׁבֵּין אָבִינוּ לַאֲבִיהֶם, שֶׁנֶּאֱמַר (בראשית כז, מא) "וַיִּשְׂטֹם עֵשָׂו" וְגוֹ'. אֵילֵי מוֹאָב יֹאחֲזֵמוֹ רָעַד. אִם תֹּאמַר שֶׁהֵן בָּאִין לִירַשׁ אֶת אַרְצָם, וַהֲלֹא כְּבָר נֶאֱמַר "וַיֹּאמֶר ה' אֵלַי אַל תָּצַר אֶת מוֹאָב", וְאוֹמֵר (שם, יט) "וְקָרַבְתָּ מוּל בְּנֵי עַמּוֹן", הָא מַה תַּלְמוּד לוֹמַר אֵילֵי מוֹאָב יֹאחֲזֵמוֹ רָעַד, מִפְּנֵי אֲנִינוּת.