It's booting into UNIX on its own.

[Ricky]

My Mac is booting into UNIX on its own. I didn't hold Apple S at startup. It started to boot up like normal, and I typed in my password at the normal login panel. I pressed return, and I ended up in a black screen with a Terminal prompt. Luckily I was able to get out of it by logging in as root and going "reboot." Started up fine second time.

Why? Anyone know how to solve this problem?