A tower spawned on my fort

[Basswar]

So, I decided to visit the Nether in search of some Nether ores to make myself some new gear. When I returned from my eventful quest and was walking back to my abode from the Nether portal I had cleverly established away from my base of operations, I noticed something odd about my small fort.

"That's odd", I said to myself. "I don't remember building a ten-storey cobblestone tower on top of and partly inside my fort."

I thought I'd go inside and see what the damage was.

"That's odd", I said to myself. "I don't remember removing the door and the secret buttons and replacing them with stone and dirt."

I decided I'd dig in where the wall was thinnest and ended up in my storage room. "Seems to be in order", I thought as I continued towards the main entrance (or what used to be the main entrance). The corridor seemed a bit on the slim side. Seeing as half of it was now stone, I think it was a reasonable evaluation. I continued down the corridor wondering where my crafting area was. I was quite sure it used to be here where all this stone is now. I'm also quite sure I had a chest full of armor and precious tools that would take me ages to craft again, nevermind obtaining the materials.

Luckily the crafting area and the materials within as well as the armor/weapon/tool chest were the only casualties of this hostile tower takeover. Unfortunately they were the most important and expensive things that could have possibly taken a hit in the whole fort.

My question in this situation is: WHYYYYYYYYYYYYY?!??

And also: Can I reverse this and/or is there anything I can do to stop huge cobblestone tower builders from invading my lands in the future?