Phex is a Java application, this means Phex runs inside a Java virtual machine (VM) on your computer. This is some kind of sandbox. Every access of Phex out of this sandbox into your system is strictly controlled and done through this VM. This means Phex alone should not be able to do anything that could make your system crash.

A clear Phex bug would be if Phex causes very high CPU utilization, which makes your system very slow and almost not responding anymore. Or Phex could just stop working, it hangs and shows just a gray window, or just disappears.

Though a system crash could still happens from time to time or from system to system, but most of the time it is related to a problem of the VM or the system Phex is running on. Phex did something, which might not even be a bug, that causes the VM or the system to go into a state that might lead to crashing your system. Sometimes it might be a VM bug, sometimes it might be a bug of a hardware driver or maybe even a system bug. Since they are not directly Phex related, these problems are very hard to reproduce or to track down. Also they usually only appear in very specific situations not always limited to just one process.
Most of the time problems are related to network issues. Phex, like every other P2P app, puts a high burden on every network related component.
Sometimes an update of the Java VM helps, sometimes a driver update helps. I don't know how these are published and available on OSX systems though.
Sometimes it helps to tune down network use.

So the next thing I would recommend you to do is update Java and hardware drivers if possible. The next time you see this crash check if your system stops responding because of high CPU (possible Phex bug) or because of a real system crash.
Have you changed anything of the Phex default configurations? As a beginner I would not recommend you to change very much there.

__________________