It seems like the spells in the HP universe are complicated and abstract enough that they must have been designed (programmed?) by wizards long ago, who added them to the laws of the universe and left them there.

Now, if I were designing a spell like the Killing Curse, I would include a little easter egg/safety mechanism: after a thousand castings, it backfires. Choose a number large enough that only a major dark wizard like Voldemort will encounter it, so it doesn't hit some minor villain and spoil the surprise. (Alternatively, rather than counting kills, count evilness, with killing a baby counting for more evilness points than an adult. That would explain why it backfired on Harry Potter, rather than some other victim.)

This is the most sensible explanation I can come up with. Or it could be that it backfired because the third through fifteenth places of the decimal expansion of the local humidity were a prime number, or something similarly arbitrary. But I would be disappointed if it was something like that. (I would also be disappointed if his parents came up with a spell that reflected it, because everyone seems convinced that no such spell is possible.)