Why is chmod a+r -R /usr/ a bad idea?
(1) "a+r" as a chmod argument? I personally don't like that style. I use exclusively octal when specifying the permissions that I give to chmod. There is no misunderstanding with octal. I encourage you to learn and use it.
(2) -R means recursive, which means that you altered the permissions of not only /usr itself but every file and every directory in /usr. Personally, I like to chmod only what I have in my sights, so to speek, the only exception being a collection of user data (e.g. a bunch of text files that I just copied over from a CDROM to a working directory on the hard drive) that I just acquired, that is currently in one specific harmless and out of the way spot and that needs to be altered as a unit. Especially, if I am root and I am working on the system, I will only change one item at a time and check things out after each change. It could be that, when you gave everything in /usr read permission to everyone, it was your system's security mechanisms that caused your problem, but that is a guess on my part. Obviously, you did create a problem.