This tidbit in the article on JD Martinez clears up a lot about what's been discussed several times on this board:
“He’d told us he’d made some changes to his swing that offseason,” says Luhnow, sheepishly. “We didn’t give him enough at bats in spring training to show us. Literally four days after we released him, he’s playing on a back field in Kissimmee against the Astros’ Triple A team and he hits three home runs. It was like, What did we just do?”