I'm guessing it was one of those freaky rare superbolts that killed so many of them over a wide area?
For a quick rundown: out of every 10,000,000 lightning strikes, only 5 are superbolts. They originate from the anvil tops of thunderstorm clouds and can carry up to a billion volts and 200 kiloamperes of current, about ten times the power of your average lightning strike..
As of now we aren't too sure if airliners can survive one of these types of lightning strikes. The only reason there hasn't been a disaster yet is because they are so rare.