strange thing happening in minecraft

Since the people on minecraft forums aren't responding, Im asking basil. Theres a TLDR at the bottom.

So i was in the nether, and when i went through the portal to go back home I found myself exiting a portal underground that i did not recognize. I went back to the nether and exited again many times, but each time i did not go back to the portal next to my house, but this strange underground portal. I decided to store all my things in a hole in the nether, kill myself so I could return home. I went back to get my stuff and brought a compass, pickaxe, and torches so i could find my way back home. I dug all the way back up, I was not far from my home. After going back to my house i tried it a few more times but still i would not be returned to the portal by my house straight from the nether. I tried destroying the underground portal, but when i entered and left the nether for a second time, i was returned to the same underground spot. Only this time it was a different portal, almost next to the one i had destroyed. I explored the area in case there were portals already existing, there were none. I tried a third time and another portal appeared ontop of the 2nd one.

TLDR: The portal in the nether takes me to random portals below ground. Even if i destroy them another one spawns.

A few things to point out, I've been exiting the nether through the same portal i entered in. I definetely did not create these portals. They did however happen to be very close to a portal I had intentionally created, but destroyed, except these ones were much further down than the one i intentionally built. Also, this is in single player survival mode and I have never downloaded mods, just played it normally.

Has anyone heard of this happening before? Can anyone explain this?