Well I just made the case that PFF is in fact subjective, so turfing it to a "well accepted" subjective source doesn't really get you where you want to go. What you're really looking for is the coaches evaluation, which is really the only opinion that counts and is of course not for public consumption.
But to answer your question, age and injuries. I would predict it happened in a similar way that it did to Andre Johnson. He made it to his last pro-bowl in 2013, in 2014 his yards went down severely (500 less) and he was cut from his original team after the season. In 2015 he played for the Colts and had a poor showing (503), and in 2017 he had 85 yards on the season and and retired.
So my question to you would be, why do you think you can take a 33 year-old-guard and expect he'll play like he did 4 years ago?