Hi Ajay,
What do you abbreviate as NLD? Non-linear distortion? In my opinion, I would say that NLD increases with the increasing input power at the transmitter terminal (that’s the main reason). This power, in its turn, is due to increase the length of fiber in a link (span, section, etc.)
In general, one can simulate non-linearities in OptiSystem by enabling them at the Enhanced tab in the Component Properties window…