ISO remained constant (244 ISO during hot streak, 235 during cold streak). It wasn't his SLG. It was his Average. He hit .191 from 8/1 to 9/26. Not to mention that he was in his first full season as a starting catcher (i.e. more than 95 games) since 2007. He was bound to be fatigued, and injured, as shown by the fact that he didn't start the last few games of the season.
He was still hitting for XBH when they landed. He got chewed up by BABIP. His BABIP for August and September was .247.