It's to do with your Mac's preferences file for Photoshop becoming corrupted.

"This could be happening as a result of a corrupt Photoshop CS preference file. If you are using OS X Tiger 10.4 on an Intel Mac then you could try holding down the Apple + Option + Shift keys as you start up Photoshop. This will provide a message which will allow you to delete the Photoshop settings file and may solve the problem."