Some of that might have to do with fatigue. Estrada is already at his usual full season of workload (minus last year) and with a bad back I'm sure he's fighting through a lot. Happ's at 164IP, the most he's ever done is 172 last year. It's quite possible these guys are simply running out of steam late in the season, they are both 33 and don't come with 10 years of 175IP type seasons.