The arrival of swallows in San Juan Capistrano on the same day each spring fascinates scientists, who continue to search for an explanation of the phenomenon.
Is the “for an explanation of” incorrect?
This question appeared on the 2009 PSAT
Grammar still isn’t my forte, but I would think that usage is perfectly correct, and it would have nothing to do with ‘an explanation for _______’
you can search for gold coins, or you can search for an explanation of why the SDMB is so awesome. 
The way the stuff about swallows is put first and then callbacked is a little confusing, but also perfectly legal I think, and beside the point of the question.
What do I win??
There’s someone on collegeconfidential putting up a pretty good fight against College Board’s decision (which is that the sentence has no error). Basically they are arguing that “explanation of” is the physical action of giving the explanation while “explanation for” is the cause of some sort of phenomena
I don’t think there is an actual error, but I think there is a subtle difference between “explanation of” and “explanation for,” and that “for” would be slightly better in this case.
If the explanation answers why something happened, or why it is how it is, then it is an explanation for that phenomenon. If the explanation answers how something happened (etc.), detailing a certain process, it is an explanation of that process.
To me, there is a subtle difference in usage:
- “explanation of” refers to explaining something continuing or innate: explanation of the muddiness of the Mississippi river.
- “explanation for” refers to explaining some event or occurrence: explanation for the I-35 Mississippi river bridge falling down.
Very subtle, and probably there are counter-examples where it would sound better the other way.
P.S. Isn’t the basic fact in this question inaccurate?
I thought the story of the arrival of swallows in San Juan Capistrano all on the same day was largely a confirmation-bias legend, with the birds actually arriving over about a week-long period (though most are on the same day). But it helps keep the tourist trade coming to repeat that. (And the whole idea of migratory birds traveling in a flock, at the same time each year, is hardly anything astounding.)