It was just plain bad luck. Occasionally rocks and other debris hit the Earth. Very occasionally, very large rocks or asteroids strike Earth. We could be unluck again some day, but the chances are very very rare that anything this enormous will hit Earth in our lifetime
Nothing really triggered it. At any one point, there are a good few things flying about in the sky. The asteroid that hit Earth did because Earth was just in the way of its path! But this doesnt happen very often at all – usually they don’t hit us!
You can, however, spot some of them with a telescope. And next February there will be one that passes by very close – though astronomers have done lots of calculations and they’re positive that it won’t hit us! Should be pretty cool to check out though if you’ve got access to a telescope!