It seems that every large company is guilty of this to some extent. I used to work at Intel and they had this exact problem when they tried to shrink the work force. The dynamic goes like this: Managers find it easier to use the performance management tool to make everyone happy rather than to follow the policies and procedures in place. So you end up with lots of quirks on an individual level -- someone wants more pay and less stock so you stick them on a low stock grant and high pay increase. Or the work in a given team simply isn't that difficult and so as your traditional journeyman gains more experience they get more expensive but not more useful- so they become at risk.
Then HR comes in to do the mass headcount cut, look at performance reviews and just cut all the people with low performance reviews. Because it's done on a mass scale of 1,000s of people they really can't do it on a case by case basis. The problem is that to safely fire staff you need to have treated them fairly and that's where it all comes out of the woodwork. You put all the data together and it turns out you've fired way more people over 50 because the organisation is a pyramid and older workers aren't value for money at the bottom of the pyramid and there's not many places at the top.
Obviously it's also true that if you can fire 10 older engineers on high salaries lots of managers will choose to do that simply because it means they don't need to fire 20 younger engineers. It's the easy option.
Not to mention the fact that if you've been at a company a long time you've most likely had your salary rise during the good years and stay flat in the bad years, either way when you get to a bad cycle again suddenly you look very expensive to the organisation.