The speed (seconds) with which the funds were redirected make it clear it was a bot.
I don't think you understand what a rainbow table is.
Somebody generated the exact same brainwallet you did, long before you ever thought of using that passphrase.
They've actually generated
millions of brainwallets, and they're just waiting for someone naive enough to use the same weak passprases and deposit money into one of their addresses.
Anything less than
16 random words is too short as a passphrase. Not a 16 word phrase from your favourite work of literature, not some TV character's 16 word catchphrase with a few simple letter substitutions and random punctuation characters thrown in.
16 words that have never before been grouped together into the same context by any human that has ever lived.
If you can't generate and remember a random passphrase this long you shouldn't use brainwallets.