Jason Kubel doesn’t really know what happened late last season. He couldn’t explain it then and he can’t now. He just knows his year could have been better. So much better. “It started good,” he said, “and then it ended real bad.” Kubel, the Diamondbacks’ projected left fielder, had 21 homers on July 22. He was hitting .300. He was on his way to the best year of his career and looked like one of the better free-agent signings of the previous off-season. But then something happened. Kubel says he’s not sure what. But for as hot as he was the first half of the year, he almost immediately turned just as cold. He thinks it was more mental than anything, a slump that got inside his head and wouldn’t leave. But he doesn’t sound certain. “I don’t know,” he said. “If I knew, I probably would have done a little better to fix it. It could be anything. There would be times when, for a game or two, I’d think that I was starting to feel it again and that I was going good. Then the next day, it was gone.”