According to the Deep Impact web site, [http://www.nasa.gov/mission_pages/deepimpact/spacecraft/impactor.html] “The impactor delivers 19 Gigajoules (that's 4.8 tons of TNT) of kinetic energy to excavate the crater.”

I know I'm being subjective, but the impact images appear to show a larger energy release than expected. (Based on off the cuff comments by one of the JPL/NASA people during the NASA TV web cast) IF that is the case, could it be the result of a “secondary” explosion(s) of volatile gases? How would that work?

Speaking of subjective assumptions, before I looked up the expected energy release on the Deep Impact web site, I would have guessed (sworn) it to be in the Kiloton range. (Based on the impact images)
Says a lot about uninformed assumptions.