http://www.denverbroncos.com/news-an...1-a830b579e15dMcManus realized last summer that he needed to remove the extra step, but when he was traded to the Broncos, he didn't have the time to work on it.
“I wanted to do it. When I was in Indianapolis and New York, they both said it would probably be beneficial for me. Even [former Broncos kicker] Matt Prater did it, I think two years before his record-breaking season [in 2013]," McManus said. "It is successful and I think there are about 50 percent of kickers in the league that have the jab step still, and then 50 percent that don’t."
I don't think it was bad mechanics, but rather which approach would work best for him. As this article says, he realized that he needed to remove the extra step, which the article says Prater did also before the 2013 season. 50% of kickers in the league have the jab step still, 50% don't. Apparently the jab step was not as effective for McManus.