Actually, the reason that Kansas City Royals star Eric Hosmer crash in the second half is easy to diagnose. He lost his plate discipline. He began to swing at pitches low and out of the strike zone more than half the time after July 1. Before July 1, Hosmer managed to lay off those low pitches that ate him up in the second half.
The heat maps linked above tell a clear tale.
No wonder Hosmer went from slashing an outstanding .305/.367/.490 before July 1 and an anemic .