Why my account keeps getting locked.

I am a little peeved at the fact, that twice now my account has been locked out, and twice i have gone to my brothers account, to find i have not logged in since the said time i logged out, so nobody could have hacked me. I have never gotten a reason for these accusations, and i am stating my grievances without the use of flaming you guys up at blizzard, the only excusable action i have taken, that i could excuse you guys for banning for, is that i log into 2 different IP addressing in a weekly basis, on weekends i'm over at my grandmother's house to raid cause my home internet is the equivalent of dial up. I am not flaming, i AM angry, but i am more so looking for an answer as to why i keep getting these lock outs.

P.S. on a side note, i could not find the forum to state my grievances/complaints, if a GM could redirect this forum there or something, or if there isn't one whatever works.