What happened
Firstly: The initial "V" of Hallow/Corruption/Crimson.
Most notably, notice the picture: picture http://hydra-media.cursecdn.com/terraria.gamepedia.com/8/8c/Contained_Corruption_and_Hallow_World.png
This gamepedia article states that:
Immediately, two diagonal stripes of the new biomes generate, replacing large sections of the World in a "V" pattern that extends from the center of The Underworld all the way to the surface.
Things might have changed with 1.3, but most likely your house is near one of the "Sides" of the V.
Secondly: When you "smash" an alter you create small pockets of one or the other:
Upon smashing a Demon/Crimson altar with the Pwnhammer (Or more powerful tools later on in the game), there is a chance of a single block of your world turning Hallowed or Corrupt/Crimson in a random place, which given enough time, will spread to form new, large areas to deal with.
So the basic gist is that your house is in a "bad"/"unlucky" place. The biomes can spread - either with time or smashed altars+time - but if your house was "immediately" in a spot, it's all location.
What to do about it
Multiple methods exist to remove Hallow/Corruption. The easiest method is a Clentaminator - which spray changes an area into a biome of choice or cleanses an area.
The Clentaminator is the fastest means of spreading Hallow, Corruption, Crimson, and Mushroom Biome, or purifying biomes of Hallow, Corruption, or Crimson.
You can also clear a surrounding area - 3 blocks to stop the spread - and then remove all the corruption to clean the area or create an artificial biome at your home. You can create Jungle, Mushroom or similar biomes that will then remove the corruption biome.
Also, check your backup folder for a save from before the switch to hardmode for a "hard" reset back. This should be in the Terraria folder, under Documents -
C:\Users$USERNAME\Documents\My Games\Terraria
Lastly, if desperate and map dependent, you can try to use TEdit - Its not been updated for 1.3 yet, but there is a chance that your world was made before. (backup your world before experimenting regardless)
As mentioned in comments, I've added these separate links to emphasize material - specifically Gamepedia - as a good spot for further research into Life-After-Hardmode. Plenty of good material on how stuff spreads, what to do about it, what not to do, etc:
http://terraria.gamepedia.com/Guide:Containing_the_Hallow/Corruption
http://terraria.gamepedia.com/Hardmode
http://terraria.gamepedia.com/Clentaminator