The water-proof watch getting smashed by a hammer isn't ironic because there's no reversal of expectations; everyone expects the watch to still be broken by the hammer. If one hyped up the waterproofing and went on and on about how nothing was going to ruin it, then it would ironic when it got smashed by a hammer.
Now, when you hear "this house has been earthquake proofed", you think "this house will not be wrecked by an earthquake" and your subconscious adds in the clause "on condition that the earthquake-proofing was done properly". If that house is destroyed by an earthquake, you'll naturally assume that the clause was violated and your expectations aren't reversed, thus making it not ironic. The earthquake-proofed house being destroyed by a tornado, on the other fact, reverses two expectations. First, it reverses the expectation that a house prepared against one disaster will do fairly well against another (though this expectation may not be accurate). Second, there are no fault lines in Tornado Alley, so one of two things must have happened: either someone living in Tornado Alley foolishly decided to proof his house against earthquakes instead of tornadoes, or someone living nearly a fault line sensibly earthquake-proofed his house only to have it destroyed by a tornado, which to his knowledge would've seemed extremely foolish to proof his house against. Either way, you expected that the earthquake-proofed was never going to have been hit by a tornado in the first place, so that's some serious reversal of expectation right there.
Or at least that's how I process it.