I know of a guy who once years ago ignored a max alt. restriction with a pack US on a 737 400. After some time at cruise level the flight attendants started falling over. (fainting). As they were walking about they needed more O2 and because there was no air exchange they became hypoxic. There was enough air to pressuize the a/c, but not enough to provide air exchange, so they were all rebreathing the same air. (170 pax + crew)
So, my guess would be that's the issue, although some of the others mentioned above could also be considerations.