Those crashes are also called Kernel Panics. Sometimes they can be caused by software. Driver problems cause them sometimes. I used to get kernel panics when I'd change my wireless driver without disabling my airport card. I have also had completely isolated incidents where I'll get one kernel panic after months of perfect functioning, and then a few more months before anything else happens. In my experience, though, the problems were all logic-board related, when they weren't software-related.