Harrison Ford is very well known in the aviation community for his enthusiasm and is generally regarded as a pretty good pilot by those who have flown with him from what I've heard.

He flies a lot of vintage aircraft. While they are held to the same rigorous maintenance standards as modern aircraft (and I can't believe someone with his kind of money would skimp by seeking out the lowest bidder for maintenance either...) the fact is that the older a machine is the more likely it is for something to fail.

Also, he is very well known for being an avid bush pilot, which is kind of the aviation equivilent of being into off-road motor sports. It is inherently more dangerous as you are often landing and taking off in very small, confined areas surrounded by trees or rugged terrain such as moutains. And rather than a nice long paved runway, you're often using fields, riverbeds, streams, snowfields, glaciers, and other unpredictable surfaces.

While most pilots will go their entire lives or careers and never have any kind of accident, it is almost guaranteed to happen to you at some point if you spend enough time flying vintage aircraft or bush piloting. The fact that he does both regularly means its no surprise that he's had a few bumps along the way.

We probably won't know the exact details of what actually happened for some time as the NTSB tends to take their time with generating the final accident report, but if it was some kind of mechanical failure like the engine quitting, then the fact he made it to a safe place without harming anyone or anything on the ground indicates he's at the very least a competent pilot.