It's probably a fungus of some sort. It would have to be examined by a mycologist to determine exactly what it is. As for "why" it is there, there are several possibilities.
First, mold spores will always be present, especially in a crop like tobacco. The only way to kill them is by autoclaving or using chemicals that would ruin the tobacco and that you would not want to smoke.
Second, the conditions need to be right for the spores to germinate. To keep mold from growing, tobacco either has to be treated with an anti-fungal or the environment has to be unfavorable to reproduction. For tobacco, that would mostly mean a low moisture content and cool temperatures. So either the tobacco wasn't treated properly with anti-fungals in manufacturing, or the conditions changed (increased moisture and temperature) to allow the spores to germinate.
Who is responsible for this mold bloom is difficult to determine via the internet.
Either way, I wouldn't smoke it.