down the pike
A pike is a turnpike. Things come down the road at you. Nothing is coming down a pipe.
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Wrong: There’s a big release coming down the pipe.
Right: There’s a big release coming down the pike.
The thing is on the road, heading toward you. That’s the whole image.
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Wrong: Let’s see what comes down the pipe next quarter.
Right: Let’s see what comes down the pike next quarter.
If you want the plumbing metaphor, the phrase is in the pipeline, and it means something else.
Where it comes from
Pike is short for turnpike: a main road, the kind you can see a long way down. American English has used coming down the pike for about a century, and the picture is the point—the thing is already on the road, you can see it from here, and it gets to you whether you want it or not.
Why the wrong one sounds right
Because it is borrowing from a real phrase. In the pipeline is correct, standard, and everywhere in the kind of meeting where this comes up. So down the pipe arrives pre-approved by a neighbour that actually exists.
They are not the same image. A pipeline is a queue you put things into. A pike is a road things arrive on. One is about what you have lined up; the other is about what is heading at you regardless.
In fairness
Down the pipe is perfectly correct in baseball, where it means a pitch straight over the plate. If you are describing a fastball, carry on.
And the eggcorn is common enough now that nobody will be confused. You will just be understood by one person in the room who noticed, and now that person is you.
Keep an eye on what’s coming down the pipe.