What did the nations hear that caused them to tremble?
Ramban #1: It refers to the Might of Hashem, that He had just displayed at the Yam-Suf - when they would get to hear about it.
Why was it at this specific time, that fear gripped the nations in advance of Bnei Yisrael?
Maharal (Gevuros Hashem Ch. 47, p. 192): Upon leaving Egypt, the Bnei Yisrael attained their lofty status above all of the nations (and they would outlast them in the end). That is why the latter were gripped with fear.
Why did terror grip specifically the Pelishtim?
Rashi (citing the Mechilta): Because they had killed the Bnei Efrayim who left Egypt early. 1
Rashbam: Because they (as well as the chieftains of Edom and the strong men of Moav, mentioned in the following Pasuk) were neighbors of Yisrael. 2
Rashi: As recorded in Divrei Hayamim I 7:21. Refer also to 13:17:2:5 .
Maharal (Gevuros Hashem Ch. 47, p. 192): The land of the Pelishtim was the closest (in terms of proximity) to Yisrael's current location. They were afraid that Yisrael would come to despoil them. (Nevertheless, the nations mentioned in the following Pasuk were even more afraid than the Pelishtim; see 15:15:3:1 .)
QUESTIONS ON RASHI
Rashi writes: "Yirgazun - Misragezin." What is Rashi clarifying with this small change?
Gur Aryeh: The verse opens with "Sham'u Amim," in past tense, whereas the verb "Yirgazun" is in the future. Rashi explains that it actually means the present tense - as soon as the nations heard [about Keri'as Yam Suf], they immediately began trembling (and they are trembling still). 1
Gur Aryeh: Rashi did not need to explain this way regarding "Natisa... Tivla'eimo" (15:12) - despite that it has the same disagreement in tenses. That verse is easily explained as follows - 'You inclined Your hand, so that the ground should swallow them.'
Rashi writes: "'The inhabitants of Pelashes' - Because they had killed the Bnei Efrayim, who hastened the 'Ketz' - the appointed time (of the Redemption).... " Why was it specifically the tribe of Efrayim who tried to leave Egypt early?
Maharal (Chidushei Agados Vol. 3, p. 185, to Sanhedrin 92b): Yaakov Avinu wanted to reveal the end of the exile 1 to his sons (see Rashi to Bereishis 49:1). Of all of the Avos, Yaakov was the most closely associated with the 'Ketz' (i.e. being the goal and ultimate destiny of the Chosen Nation]) - and thus he knew its secret. 2 Yaakov passed it on to Yosef, his primary offspring; 3 who in turn taught it to Efrayim, his foremost son. That is why the tribe of Efrayim sought to hasten its arrival more than others.
Did Yaakov want to reveal what would happen at the ultimate end of exile, i.e. Mashiach's arrival; or did he refer only to the end of the current exile, in Egypt? See Bereishis 49:1:1 .
Review Bereishis 49:1:1.2:1 , and the ensuing questions on that verse from Maharal. Yaakov was forbidden to reveal the end, because the times of Mashiach will be like a new world; it was not fitting that its secrets be revealed [until its time]. The secret was revealed to Yaakov only, due to his inner, holy power.
We may infer from Maharal's comments here, that also Yosef, like Yaakov, reached a level worthy of this revelation; also refer to Bereishis 37:2:1.09:1 . (Upon Yosef's passing, he alluded to the end of the Egyptian exile in his words, "Pakod Yifkod" (Bereishis 50:25); see our commentary to that verse.)