I would guess that you used the same name for the new world as the old one?
And also that in your old world you had a Nether portal unprotected outdoors in the Overworld?
There is a bug or glitch or whatnot, if you delete a world and create a new one with the same name some of the chunks from the old world can show up in the new one. Apparently that works with worlds that are "deleted" due to power outages as well.
And leaving Nether portals that mobs can get into is a bad idea since the mobs that travel through the portal while you are elsewhere in the Overworld end up in a Nether that isn't loaded and so can't despawn or wander off and so are there waiting for you to show up.
As far as I understand things, when you reuse a world name Minecraft uses the same directory, its supposed to delete all the chunk files but it doesn't check if it has done so before letting you close the game. Of course in your case it didn't actually delete any chunks, having the computer go down must have messed up the file header or something so MC didn't recognize it but some or all of the chunk files were still there undamaged and so those chunks were there waiting when you entered those areas of the world.
Obviously you built the new portal in the Overworld within 1024 blocks or so of the one in your base and so it took you to your original portal in the Nether, hence all the mobs waiting for you and the portal taking you back to your base.
Did you use the same seed?
If not then you'll be getting unsightly cliffs between the old and new chunks.
Glad you got your base back!
There is a thread for help with corrupted saves, I don't know if anybody can do anything to get more of the world back after you have played it again, I haven't really read that thread much.
It might be a good idea to make a backup of the world save directory before playing that world again just in case there is stuff missing that you'd want to get back and somebody is able to help with that.