Optimize table is run automatically on a weekly basis. Depending on the size of your visit table, the table may be locked for a very long time and kill your database and web server.
The table is locked for the time it takes to create a copy of it, which means that no writes can happen, which ties up the database with INSERT queries and ties up all the webserver processes serving mint.
It would be really great if you could configure the time of day during which the optimization happens and/or make it a manual task only so that it doesn’t run during heavy load and DOS your site.