Why do operating systems freeze?
A "system freeze" is normally due to the system running out of some resource and spending it's limited resources left trying frantically to fix up the mess. I.e., too much CPU load, has no processing power left for the user. Memory overload, keeps trying to free memory and gets nowhere, no time left for the user (and no memory to load programs).

It might also be a deadlock (some tasks inside the system are holding resources others require, and are waiting for still others to be released, but that will never happen as everybody is waiting for somebody else's resources).