Ubuntu 9.04 VM crashing Mac OS X Snow Leopard

So, here's the story. I use VirtualBox 3.0.8 (downloaded directly from virtualbox.org) for virtual machines, with Mac OS X v10.6 (snow leopard) as my host OS. I have many guest operating systems. One of them however, does not work. When I open it up, Mac OS X restarts. It is really very weird, because it didn't do that before. This particular guest is Ubuntu 9.04. My dad had it and it worked for him, and he successfully upgraded to Ubuntu 9.10 and that works. I only tried to boot into my Ubuntu VM to upgrade it to the latest version. So my questions are: why does it crash Mac OS X? How can I fix it?

Actually, maybe I should ask this same question on the VirtualBox Forums.

Thanks in advance.

I just booted up the VM and it works now. But why did it behave like that?

Your guess is as good as ours. Right click your VM from the list and select "Show Log". There might be something in there that can give you an understanding of what may happened.