kitcken sink drain

We have a problem,there is a pipe sticking out of the back of the house that every time we run the kitchen water in the sink it drains out of this pipe.It is an older home and the pipe is aprox. 6" around ,i have never seen this before ,what is the pipe and why is the water draining there?
thsnks for any help.

How long has this been happening? A pic would be helpful

It could be a plugged drain in the midst of a previously botched plumbing installation of your kitchen drain.

I guess it would also depend on what part of the worl you live in.

not sure exactly how long its been happening ,we live in northern california.I looked under the sink and everything looks normal to me ,the drain leads out through the wall under the sink ,this pipe stick out about 6' and is at an upward angle .i will try to get a picture of it .

It almost sounds like a vent that was not taken to the roofline. It may have been terminated way below the flood level rim of the sink. If this is the case, you could try running a drain snake through the exterior pipe where the water is coming out of. It may be plugged or clogged.