I'm not sure where I found this or who wrote it; like many of these sorts of things it may well be the product of a great many very clever people. Enjoy.
Walt Whitman: To cluck the song of itself
Jack Nicholson: 'Cause it (CENSORED) wanted to. That's the (CENSORED) reason.
John Paul Jones: It has not yet begun to cross!
Ralph Waldo Emerson: It didn't cross the road; it transcended it.
Aristotle: To actualize its potential.
Pyrrho the Skeptic: What road?
William Shakespeare: I don't know why, but methinks I could rattle off a hundred-line soliloquy without much ado.
Thomas Paine: Out of common sense.
T.S. Eliot: Weialala leia / Wallala leialala.
Groucho Marx: Chicken? What's all this talk about chicken? Why, I had an uncle who thought he was a chicken. My aunt almost divorced him, but we needed the eggs.
Karl Marx: To escape the bourgeois middle-class struggle.
Mark Twain: The news of its crossing has been greatly exaggerated.
Mr. Scott: 'Cos ma wee transporter beam wasna functioning properly. Ah canna work miracles, Captain!
Robert Frost: To cross the road less traveled by.
Sigmund Freud: The chicken obviously was female and obviously interpreted the pole on which the crosswalk sign was mounted as a phallic symbol of which she was envious.
William Wordsworth: To have something to recollect in tranquility.
Caesar: To come, to see, to conquer.
Bill the Cat: Oop Ack.
Rene Descartes: It had sufficient reason to believe it was dreaming anyway.
Albert Einstein: Whether the chicken crossed the road or the road moved beneath the chicken depends upon your frame of reference.
Leda: Are you sure it wasn't Zeus dressed as a chicken? He's into that kind of thing, you know.
Zsa Zsa Gabor: It probably crossed to get a better look at my legs, which thank goodness are good, dahling.
George Bush: To face a kinder, gentler thousand points of headlights.
Epicurus: For fun.
T.S. Eliot revisited: Do I dare to cross the road?
Zeno of Elea: To prove it could never reach the other side.
Emily Dickinson: Because it could not stop for death.
Howard Cosell: It may very well have been one of the most astonishing events to grace the annals of history. An historic, unprecedent avian biped with the merity to attempt such an herculean achievement formerly relegated to homo sapien pedestrians is truly a remarkable occurrence.
Gregor Mendel: To get various strains of roads.
Sisyphus: Was it pushing a rock, too?
Salvador Dali: The fish.
Lee Iacocca: It found a better car, which was on the other side of the road.
Henry David Thoreau: To live deliberately...and suck all the marrow out of life.
Mae West: I invited it to come up and see me sometime.
Joseph Conrad: Mistah Chicken, he dead.
Gerald R. Ford: It probably fell from an airplane and couldn't stop its forward momentum.
Gottfried von Leibniz: In this best possible world, the road was made for it to cross.
Candide: To cultivate its garden.
George Washington: Actually it crossed the Delaware with me back in 1776. But most history books don't reveal that I bunked with a birdie during the duration.
Dylan Thomas: To not go(sic) gentle into that good night.
David Hume: Out of custom and habit.
John Milton: To justify the ways of God to men.
James Tiberius Kirk: To boldly go where no chicken has gone before.
Ernest Hemingway: To die. In the rain.
Gilligan: The traffic started getting rough; the chicken had to cross. If not for the plumage of its peerless tail; the chicken would be lost. The chicken would be lost!
Thomas Dequincy: Because it ran out of opium.
Socrates: To pick up some hemlock at the corner druggist.