So Blaze Injun brought up an interesting idea and I'm posting it here for public digestion: if a large gas giant is hit by a comet then there is a small (5% chance) that it stimulates it enough to cause it to become a brown dwarf.
Now here are my additions:
This in turn has a chance to form a new small terrestrial planet (10% chance) and/or disrupt the orbit of nearby smaller planets, causing varying degrees (1-10 chance of catastrophic changes; 11-25 of major-bad changes; 26-50 of minor-bad changes; 51-95 of no changes; 96-99 for minor-good changes; 100 for major-good changes).
What do you think? For the bad/good changes I am thinking pulling/pushing planets out of/into habitable zones, scattering asteroids about, messing around with moons, etc.