This has happened to me several times now in the past fortnight. I hit the BPR Render button and my computer literally turns off.
I’ve got a 27inch iMac, 8GB Ram 2.8 Ghz Intel Core i7 OSX 10.8.2
It doesn’t happen all the time, but it just happened again with a 1.4 Million poly mesh and shadow rays set to 1.
ZBrush itself isn’t turning your computer off. More likely, one of your system’s components is not responding well when your system is taxed and the computer shuts down. There are a number of possible causes: a heat issue, RAM or CPU problem, etc. Is there any kind of event log that you can access after restarting the computer, giving the cause of the shut down? That sort of thing would point you to where the issue is.