What triggers disasters in SimCity?

The achievments which unlock disasters seem to point to particular city conditions that may make a disaster more likely. For example, here are the achievements that are earned which unlock the player ability to trigger a disaster.

• Dug Too Greedily and Too Deep - Mine 100 tons of coal or raw ore in your city to gain access to the Earthquake disaster
• What Goes Up Must Come Down - Have 200 tourists arrive on flights at the Municipal Airport in a day to gain access to the Meteor Strike disaster
• Laboratory Outbreak - Add a Diagnostic Lab to a Hospital and have 15 Sims die in a day to gain access to the Zombie Attack disaster
• What's Cookin'? - Burn 100 tons of garbage in an Incinerator at the Garbage Dump to gain access to the Big Lizard disaster
• We Are Not Alone - Ship any resource to a Space Center Great Work to gain access to the UFO Encounter disaster
• Blowin' in the Wind - Have 24 Wind Turbines at Wind Power Plants in your city to gain access to the Tornado disaster

However, I can say from personal experience that I have had every one one of these disasters happen without any of the potential conditions listed by the achievement in play. I've had an earthquake with no mine, a meteor strike with no airport, a zombie attack with no Lab, etc.

This leads me to believe that disasters are truly random, and that you may just be luckier than your friends.

Without an official post from EA/Maxis we will never truly know. As I said earlier, I have experienced disasters of all types in small or large cities, dirty or clean cities, cities of different specializations, without a discernible pattern.