Some teams, and players, just seem to do this. I have guys every season who do the same thing and it is not because of durability or any other rating that I can tell. A catcher of mine hits about .385 in the first half and ends the year at about .325, every season. A 3B of mine hits about .220 in the first half and ends his year at about .300, every season.
I have pitchers that skittle around for the first half and then dominate in the second.
Some teams charge out of the gate and play .650 or higher ball for a while then I zoom past them to win the division, every year. And the wierd thing is, the same teams, and same players seem to do this every season. I can't figure it out, but I see the ratings and know it is going to happen again.