The Taliban are mostly a bunch of illiterate religious "students" (ethnic Pashtuns from Pakistan and Afghanistan) out of the madrassas of Pakistan. They most certainly are not the traditional elite of Afghan or Pakistani society, which more or less sided with the Communists during the Cold War. So to answer your question, it most certainly is a grassroots movement; however, it was bolstered by active American involvement in that region. When they first came to power, the population was largely sympathetic and supportive. Today, they are not.