The New Fowler's Modern English Usage claims that this expression first appeared in Shakespeare's Macbeth. "Fell", sez Fowler, is archaic for "deadly" and "swoop", sez Fowler, is archaic for "swoop" -- as in a bird of prey diving on its victim. Fowler finally sez that the sense of danger which the phrase once carried has diluted with use such that it now means only "all at once" or "suddenly."