No great surprises in the article. About once every four years on average seems about right for me, and I've never been to Oregon.
I've been off a few times in 50+ years of cycling, and the majority have been down to either road hazards of the sort mentioned, slippery roads (which don't seem to be mentioned) often owing to oil on the surface, or my errors, or a combination of the two. A couple have been because of drivers' errors. Fortunately none have caused me a serious injury.
Many could have been avoided by better road maintenance or design, as is said for the survey in the article.