A theoretical categorization of the onset of tearing modes in tokamaks is presented using DIII-D equilibrium reconstructions as initial conditions in the NIMROD nonlinear three-dimensional resistive magnetohydrodynamic code (Sovinec C.R. et al 2004 J. Comput. Phys.195 355). The onset mechanism of tearing modes are categorized into three types: spontaneous, mixed and forced depending on the importance of linear instability versus forced reconnection. The physics of the early evolution of growing tearing modes in simulations with time varying linear instability drive, nonlinear coupling between modes and neoclassical bootstrap drive are compared to experimental data to explain this qualitative categorization. Important effects, such as rotational shear and thermal anisotropy, are included and confirm that this categorization is consistent with the experimental observations in DIII-D.