Two possible nucleation mechanisms for threading edge and screw dislocations during the physical vapor transport growth of SiC have been investigated. First, growth over intentionally deposited carbon inclusions led to an edge and screw dislocation density orders of magnitude higher than the surrounding crystal. Second, seeds with mechanical polishing damage have been shown to lead to a dislocation density nearly three orders of magnitude higher than seeds that were hydrogen etched. A new linear step source has been observed and correlated with an increase in the dislocation density.