TPS51367 PGOOD Question

For almost any IC as the input voltage starts to rise from zero, there is no control over the internal functions as there is not enough input voltage for the internal circuitry to work properly. As the input voltage rises to about 0.6 V to 0.8V, some internal PN junctions may become forward biased and allow some voltage to appear at output pins. As the voltage continues to rise up above about 1.2 V, the internal circuitry becomes aware and can control their functions such as PGOOD and force the outputs low or high as required. This is what you are seeing in your diagram.