I think I've figured out the problem and it's not what you think :-)
let me give you a little bit of background:
In order to crate a this particular wallet I was using "Brain wallets" (which is basically a sha-256 of a text), but I was using simple words like "black" and "white".
I think this simple "words" are part of a databases of "simple" KEYS, which could be easily generated from any source of "public common passwords".
That's because you didn't mentioned it in the first place.
If so, anyone would figure that it's the problem since it's the very issue why brainwallet was discontinued.
For reference, there's list of cracked brainwallets:
http://bitcointalk.org/index.php?topic=4768828.0