Hmm, very interesting. I have two hyoptheses:

1.) Since Don beat me to the keyboard, I would venture a guess at the
Baccus theory. That is, they do nest in close quarters and if the road
were near a bank they were nesting in, it's feasible that cars could be
killing them left and right as the birds fly in and out of nesting holes.

2.) Aerial insecticide use after Hurricane Hugo caused brief but
widespread Tree Swallow mortality in coastal South Carolina. It could be
that crop-dusting may have somehow caused this, if their prey (or perhaps
the birds themselves) were sprayed with insecticide and promptly consumed.

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Michael R. Smith
Univ. of Washington, Seattle
whimbrel at u.washington.edu
http://salmo.cqs.washington.edu/~wagap/mike/mike.html