Exactly, it never "just happened." An immeasurable amount of things happened over billions of years that gradually led to what we see before us today. There are a couple big events we can point to in some cases, like glaciers carving out areas in Yosemite and then melting away, but even those things took millions of years to happen. The life that exists in that photo is a little different than it was a million years ago, and way different than it was several million years ago because of a gradual change in the genetic makeup that happened to meet the ever changing habitat and allowed each lineage to successfully evolve. And, as each form of life changes, each part of the ecosystem may need to change slightly to until it balances again.

Even if a volcano erupted and destroyed all the life that exists there, it still would have not "just happened." It would have been the work of millions of years of volcanic movement beneath the surface, on top of all the billions of years of things that happened before that.

I think the biggest issue is creationists hold so tightly to the idea that god created everything suddenly and in its entirety that when we start to talk about geology and evolution they just hear us saying it happened suddenly for no reason. Yes, we might be saying that everything happened without a reason in regards to lacking an intended purpose, but we aren't saying there wasn't a reason. The reason was billions of years of things that led up to that moment. Nothing sudden about any of it.

As far as the big bang stuff is concerned, we aren't saying it happened suddenly for no reason, either, we are just saying we don't know what led up to it since we can't see before or outside of our universe.

Sorry for preaching to the choir on this, but I wanted to type out my irritation.