I never have heard of this being mentioned as a feature. Occasionally when returning from the nether I will come out through some random, self generated portal I never made. Why is this?
Why do minecraft portals sometime spawn themselves?
Nick Capesius
Say you go into the nether and your portal outside is destroyed, when you come back out it will spawn another one. Also, if your portal in the nether is destroyed and you try to re-enter, you will spawn another one.
There are also occasionally random ones created like the ones I think you are describing. They can be caused by a portal being blocked or for no reason at all, I believe.
Not much you can do to stop it as far as I know.
Kraise