It's probably a combination of three, but here were some very interesting stats from that article.
In his first eight starts, 13.9 percent of groundballs hit into play were “well-hit” balls, which is above the league average of 12.9 percent. But in his last six starts, only 6.9 percent – a mere four of his 58 grounders in play – were “well-hit”.
Niese also appears to be benefitting from a vastly improved infield defense in his last six starts. According to Baseball Info Solutions, in his first eight starts, the infielders combined for a plus-minus rating of minus-2.4, which means that they made two fewer plays than the average infield would have made; in his last six starts, the infield was plus-6.3, as they have combined to make six plays more than the average infield.
My Take: This was very informative and highlights how much the improved defense helps a ground-ball pitcher like Niese, and the reason why I believe Tejada should still be the starting 2B when Castillo returns.