The reason is simple. Think about what a screensaver does. It s constantly counting down from the last time you hit a key or moved the mouse. If it reaches the magical wait number, it barges in and does it s little song and dance. The problem arises in that barging in behavior.
Some programs don t like to be interrupted.
They conflict with the screensaver causing a nice freeze-up. Whenever someone calls me complaining of constant freeze-ups, the first thing I recommend is to turn off their screensaver. You d be surprised at how often that fixes their problem. My monitor turns itself off after ten minutes of non-use. Wait, you ask.