With SSI they track withdrawals and require an accounting of where the cash went, along with supporting documentation.
Every two years they do a review where they require all your bank statements (or verbal agreement to contact the bank directly for them) and especially look at any cash withdrawals, then require you to explain what the withdrawal was for and provide documentation to prove you did with it what you're telling them you did.
Furthermore, even when you provide a full accounting of everything, often times this is not placed correctly in your record. This means you end up needing to provide all the information multiple times. 6 weeks ago I received a letter that, as of next week, my income was being stopped completely--all because I bought a van in November 2014 with a loan from two friends. I had given them all the information multiple times, in fact the notes explaining it were right there on record in the paperwork they sent to me--and yet it was filed incorrectly, and when my review came up it was flagged, and my income was stopped. I had to contact my congressman to get it sorted out. Just like I had to two years ago, too. Different issue that time.
That's what happens with a single instance of withdrawing a bunch of cash. They know that once you withdraw it, they can't track it, so they force you to provide an accounting of where it all went.