Like what I've said on my previous reply, it's most likely just them asking for proof that you're not selling some illegal stuff, or they're going to try to tax you.
Since when banks collect taxes, a bank isn't a governmental institution!
@OP
You worry for nothing and @teosanru gave a hint. Since you mentioned you're from the EU, banks use algorithms to detect suspicious activities. In Europe, the threshold is about 5000 during the last 30 days. Once your account is flagged the bank is supposed to do the first check and then supposed to report your bank account to the financial authorities, then it is up to them to make the necessary investigations. Some don't bother to do the first verification (since it's automated), in your case, your banker did it.
You don't have to lie, it never pays btw. On the opposite, if you are able to explain clearly what you are doing (and if you can also provide evidence) you have nothing to worry about. There are thousands of accounts reported in the EU but in the end, just a very very low percentage of the accounts are truly doing something shady. (99% are false reports).
Another point.
It is possible that your bank thinks you have commercial activity and as personal accounts are not allowed for that (banks hate it). They would like to be sure (in case they can sell a pro account). You can tell them it isn't a regular activity just an extra income, they will be fine.
If you use this account just to do your LBC stuff, it's easy for the bank to think you're running a business (No bills paid, no recurring payment, no card transactions etc)