Don't know why it happens, but does the screen just go black or does the screen saver start?
Maybe this thread will help: https://discussions.apple.com/thread/1011009?start=0&tstart=0
SMC & PRAM reset: https://discussions.apple.com/docs/DOC-3603
Maybe this will would suit as a workaround if the resets don't work. If it's the screen going black you could try adjusting the Display Sleep time when you want to watch a DVD:
System Preferences>Energy Saver>Display>Never.
If it's the Screen Saver: System Preferences>Desktop & Screen Saver>Screen Saver tab and set it to 'never' from the 'Start after' drop-down menu.
Then remember to change the settings back after watching.