Headcases ? No, just human.
It's human to be superstitious like many players are. It's human to have some slumping periods. When Ryan Braun decided (a year or two ago) that he was going to focus on hitting for high average instead of power, that was a conscious decision he made and one that's very human, and entirely understandable from a contract perspective. So you may see hitters hitting more ground balls and line drives instead of fly balls to try to improve their average, or vice versa, and you may see hitters slumping occasionally and not demonstrating a good eye. But once they choose an approach (and data backs this up), players stick with it regardless of who is in front of them and who is behind them. If they are like Guzman or Guerrero and can't (or don't want to) walk a ton, they don't walk much whether or not a great hitter is coming up next. If they are "Greek God of Walks" Youkilis or Adam Dunn, they walk a lot no matter who is coming up next. This falls under the same category as clutch hitting: at the major league level, players treat every at-bat more or less the same way, unless instructed to do otherwise by a manager.
(Besides that, walks [which is what the idea of protection is largely based on] are different from things like average vs. power, because they are nearly
always good. Sure, a walk isn't quite as good as a hit most of the time, but not only are you guaranteed to get on base, players who walk a lot have higher BABIPs, more pitches per plate appearance [which, again, results in higher BABIPs], score more runs, etc. Walks are good for hitters and bad for pitchers. Every single major leaguer knows and knew that [even Nolan Ryan]. In this day and age, that is true more than ever.)