That is very uncommon occurrence. If I were you I would contact SIPC http://www.sipc.org/ and FINRA http://www.finra.org/ immediately and fill in a formal complain with them. At least that. Immediately.
Could be a clerical error. Some employee logs in, thinking you are bob smith, and there are 100 bob smiths at the brokerage...etc I sometimes get insurance checks from people with a similar name on the same insurance coverage. They obviously inputted the wrong data. Mistakes happen.
Did the withdrawal show up as a regular debit, as with a normal withdrawal, or was $10,000 just "missing" after accounting for changes in daily P/L?
Does TOS not require you to authorize such transfers using a security device, in addition to your password, ie, a two-factor authentication system? Is the "strange" bank account in your name or someone else's name?
If you knew anything about taxes and if the funds really were stolen you'd know that what you just said is absolutely, positively FALSE! Don't make comments about the tax code unless you know it inside and out and if you don't do the research to find the correct answer. The OP would not be liable for these penalties if he has an accountant who has a shred of knowledge.
OK smart guy, why don't you post the IRS code that says funds withdrawn from IRA account are exempt from taxation and penalties and what form of proof is required by the IRS for the exemption.