The expectation values for stretch and momentum in O-H and O-D bonds of the H-O-D molecule subjected to UV pulses effecting selective cleavage of these bonds have been analyzed to decipher the mechanistic basis of selective dissociation. Fully quantum dynamical calculations using both the ground and excited potential energy surfaces with different initial states and UV pulses reveal that prior stretch in a bond before transferring it to the repulsive first excited-state ensures preferential dissociation of this bond. The sampling of large stretch values and a quick downhill motion in the channel corresponding to dissociation of the prestretched bond on the upper Surface are seen to underlie this preferential dissociation.