Konnon,
30 grand? I bet you set the Fed spidey senses just tingling! Another thought as far as why they locked in on you would be:
A:Money Laundering-The process by which criminal enterprises attempt to take money made by, or for, illegal means to look legit.
By depositing a large amount of money and then immediately withdrawing it it looked like you were doing some sort of wire transfer scam or "washing" it. It could look like it was dirty money from a dozen different places-Terrorists, Drugs, Mob, Girlscouts (I know there's something addictive in those cookies and I'll figure it out after just one more....

).
B:Nigerian Scam-Although it's unfair to call it the "Nigerian Scam" since it's really caught on in alot different places. The scam comes in various forms, but a common one is to someone cashes a large check, money order, money transfer, or multiple checks adding up to a large amount, into an account. Now these are either from people that have been scammed and don't know there money is on it's way to a country with very liberal banking laws, or the money order/checks are counterfeit. You might think that in todays modern era counterfeit checks would be caught in an instant, well sad to say the answer is "no". Often the bank accepts the check/money order as legit until the paper reaches it's destination where it is found to be from a closed account, false account, or fraudulent account. All that takes a few days.
Then it comes back and somebody is out of a lot of money (usually the bank or some fall guy).
I'm glad to see you came out of it intact. It would be horrible trying to prove your innocent when your not even directly being accused of anything just some vague descriptions, suggestive questions, and no one that can comment on it directly!