There are a number of possibilities. First of all, note when the ages dropped off dramatically - right after Noah and the flood. The cut in half for Noah's sons, then half for their sons, and then finally the next generation - Abraham - lived close to what we'd consider a normal life.
It's quite possible that the earth underwent dramatic changes with the flood. When the skies were opened up, it may have removed a protective barrier which allowed more radiation in.
Another possibility has to do with atmospheric pressure. Scientists believe that the atmospheric pressure was about double what it is now. I don't understand all of the science behind it but greater pressure seems to help with healing. This is why some people sleep in hyperbaric chambers to recover from an injury.
The third possibility has to do with genetics. In short, Noah was the last to have the longevity gene. His children had it, but only half because they only got it from their father. The next generation only had one quarter of it. This would make sense from the ages that I mentioned earler.
In truth, we can't be certain but any or all of these three are possibilities.