To get to the other side.
You may think this is a joke, but we submit that it is a valid question. Why do chickens cross the road? Particularly around dinnertime? In fact - why do chickens go anywhere? Do they have a purpose in where they decide to go? Is there any decision process that occurs at all?
Mike has noted that one way to tell that chickens really aren't thinking very much is that they will peck at just about anything to see if it can be eaten. Marianne has noted that another way to tell that chickens really aren't thinking very much is that they continue to return to their owners even though they are not given any feed. They obviously haven't thought through the ramifications of their chicken-human interactions...
Here's another question: Why did the goat cross the road?
Goats appear to be slightly disoriented, indecisive animals with an apparent thrill-seeking gene. You'll spot a goat beside the road some metres in front of the vehicle, and it's looking at you, not sure what direction it wants to go. Then, just as you are about to come upon it, it bursts into action across the road in front of the vehicle, narrowly escaping yet another brush with death.
Well they really seem to want to get to the other side - and they don't seem in much of a hurry to get there...
As a side note, here is a video we captured of a man and his wife chasing a chicken in order to present it to us as a gift, as their guests of honour! (Click link if you're viewing this post over email).