This is the real cause of most cookie deletion. (EDIT: Many) browsers are configured by default for ~1000 cookies, and all these ad networks and data providers setting their own cookies quickly overrun that. It doesn't help that google Analytics sets separate (first party) cookies for each website you visit.
That 1000 cookie limit becomes a sort of FIFO, and that's why we always seem to get logged out at weird times for no reason.