Not really a coincidence. Happ, Stroman, and Sanchez are GB pitchers which decrease pitches per inning and except for Happ and Stroman in the last 2 weeks or so they have pitched great which will also decrease pitches per inning. Estrada is there because his changeup and ability to throw it whenever induces a lot more swings (pitches in the zone) but a ton of pop ups and easy fly balls.