I agree, many of our errors are caused by just plain bad defenders, but many are also caused by mental blunders and a seemingly lack of focus and concentration. I'm not just talking about plays that are called errors, either.
I know it can't be proven, what is going on in a player's mind before a ball is hit or thrown to him, so it's all speculation, on my part, but these guys seem to have their heads up their asses. Even some of our "good defensive players" have blundered plays. For example, Rafaela has 3 OF errors.
We throw to the wrong base. We double hitch, before throwing. We fail to get our body and feet in the right position to make a play, and much more along these lines. Yes, bad defenders don't do these things well, but these mistakes seem more mental than physical, because we do see them do the plays right, sometimes, so they do know how to do it correctly. They just don't, too often, and I chalk much of it- not all or maybe not even half- to a lack of concentration and preparedness.