Why did social security deposit extra money in my account on oct16?
I don't know, but this is normally the time of year that Social Security processes automated benefit adjustments. So, the payment you received could be back pay due as a result of a retroactive rate adjustment for any of a variety of reasons (e.g. delayed retirement credits, additional earnings, etc.).
Direct deposits are often received up to a week or more in advance of the explanatory notices that Social Security sends in the mail, so I would wait a while to see if you receive an explanation. If you don't, you should probably check with Social Security.