In general, I'd say because it is an old App that is a serious POS. In specific, I have no idea.
It's a PowerMac G4 purchased in 2003. Actually, I love it and it gives me very few problems. I still use Windows everyday and have two Windows PCs in addition, but this computer really exposed me to how awesome Macs are for reliable use. I can count the number of times Safari has ever closed down on me on one hand. I'd need two hands for how many times IE shut down on me last night alone.
I........ IE, but it constantly gets an error message and closes down after a few pages. Any idea why.......