Not heard 'down the pike'? You need to get out more bertie!
Maybe I overstated the amount of confusion. I meant that I have often heard people confused by this expression. Actually if you google 'down the pike' you will see that this confusion is widespread.
I'm guessing it's another one of those cases where an expression evolves as it crosses cultures. Like Americans saying 'I could care less' where a Brit would say 'couldn't'. So when an unfamiliar phrase about an old european weapon came into a culture that used the same word to mean road, they assumed that it meant coming down the road (or pipeline).
I don't know this for a fact though.