Banks must file a CTR (Currency Transaction Report) when you deposit more than $10,000 in cash at one time. Something to do with money laundering.
A currency transaction report (CTR) is used in the banking industry to monitor and report cases of potential money laundering.
www.investopedia.com
True story - because of my business ventures over the years, I am familiar with CTRs and try to avoid them whenever possible, not because I am doing anything illegal but because I try to avoid government reporting whenever possible. A few years ago when I was handing over the reins of treasurer of a local organization I had been involved in, I told the new person to avoid cash deposits of $10,000, explaining the CTR process. This organization did have a couple times a year where fundraising would generate this much cash at once. The person taking over from me said it didn't bother her to have any CTR reports filed since she had nothing to hide. Low and behold, a year later when her security clearance was coming up for renewal, the investigator asked her about these large cash deposits, in an account that wasn't hers! But hey, what do I know - I'm just a dumb farmer right?