Why does it say "v'Af Elokim Alah Vahem"?
Radak: This is like "v'Af Hashem Charah va'Am" (Bamidbar 11:33). "Alah" applies to anger - "Alah Ashan b'Apo" (above, 18:9).
Malbim: Do not say that they died naturally, that the Slav was hard to digest, especially since they had not eaten meat for many days. "B'Mishmaneihem" (the strong ones) died; a harsh food would kill elders or people of a weak nature. Here, the fat, Giborim and youths died, and not the elders!
Who are "b'Mishmaneihem"?
Radak: They are the healthy and strong ones. This is like "Kol Shamen v'Ish Chayil" (Shoftim 3:29). The strong ones and youths died from Hashem's anger. (Also refer to 78:31:1:2.)
Who are "Bachurei Yisrael"?
Rashi: They are the Nivcharim (choicest ones).