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First...look at the walk rate.
his peak, he was a 1.6 to 1.8 BB/9 pitcher. Starting in 2004 (and in 2000 when he was also injured and ineffective for part of the season, thereby costing him touch on his pitches), he started walking 2.5 to 2.8 per game.
To go with that, his flyball rate has increased (he was a flyball pitcher for most of his career, but it's getting worse, hinting at his loss of ability to keep the ball down), which along with an increased HR/Fly has resulted in a massive uptick in HR rate.
I'm not an athlete, but I am inclined to believe an old guy when he bemoans that he "justs can't get the shot there" anymore as he ages trying to play racquetball. Same thing applies to Moyer's pitches.