Most of legitimate financial activity forms follow well known patterns. Illegal activities often follow different patterns. It's reasonable for a bank to flag suspicious accounts, even though you'll always have some false positives - because if it looks like money laundering, most of the time it is.

Also I don't know if this works with banks (having no personal experience here), but I remember back from the time when early Kickstarter projects got their PayPal accounts frozen, that the rule of thumb was: if you're going to do something on your PayPal account that looks weird/atypical, it's a good idea to call them in advance and tell them about that.