I came across this in a text book yesterday and it's been on my mind since. To be fair, it's not the best sentence anyway, but it did make me think.
"If a tornado hit Rex's house, it would destroy it."
If you swap the clauses - The house would be destroyed, if a tornado hit it.
Why does the main clause turn to passive voice when its position is swapped? I've been racking my brains, but I don't recall seeing this before with conditional statements.
The only reason I can think of is that the double use of IT in the original main clause doesn't work when you swap it round. Thus for it to make sense you need to talk about something directly.