I don't know why companies do the "escort you out by security" thing, but I doubt there is a great deal of logic to it, as I have observed that even at a given company, some people are treated like that and some aren't, and there's no sense to it. It's never happened to me, but a former boss of mine was laid off that way, and nobody admitted to knowing why. Yes, he could have destroyed a lot if he'd wanted to, but so could a less senior employee.