The initial phase of defect propagation in broad area diode lasers, which are affected by the catastrophic optical damage (COD) effect, is studied. The decay of laser power within the first several 100 ns is found to be determined by defect propagation. When analyzing different device designs, a correlation is found between defect propagation velocities and thermal resistances of the materials vicinal to the quantum well, being the main heat source. The findings are confirmed by direct inspection of the defect pattern in opened devices.