The Mekhilta applies the same logic to Moab that it applied to Edom. The verse says "the mighty ones of Moab were seized with trembling," and the rabbis ask the same question: why? Israel was explicitly forbidden from taking Moabite territory.
God had issued clear instructions (Deuteronomy 2:9): "Do not antagonize Moab." And regarding the children of Ammon, the command was equally explicit (Deuteronomy 2:19): "When you come opposite the children of Ammon, do not antagonize them." Both nations were protected. Their lands were off-limits to Israelite conquest. So the trembling of Moab's mighty ones demands an explanation.
Once again, the Mekhilta identifies the cause as worry over contention — not fear of conquest, but anxiety about the inevitable friction that comes with proximity to a massive migrating nation backed by supernatural power. The Moabites watched the Red Sea split. They heard about the plagues in Egypt. They knew that the God of Israel had rewritten the laws of nature on behalf of these former slaves.
Even with a divine guarantee that their borders were safe, the Moabites trembled. The Mekhilta understands that political guarantees do not eliminate existential anxiety. Knowing you will not be conquered is one thing. Watching the conqueror of the world's greatest empire march past your doorstep is quite another.