Maybe because the DVD drive is broken?
It would be nice if you could offer some more information, to eliminate false guesses by other posters.
For example, what Mac and Mac OS X version do you use on that Mac that so blatantly spits out the DVD?
What kind of iWork version do you have, that you want to install, license-wise?
Because the Mac does not know, it has a DVD inside that was used five times before or even 500 times.
And when exactly happens the spitting part?