Question

why does nylon clothing crackle as you undress??

Answer 1

the movement of our body build static electricity and nylon is the perfect media to hang on that charge .then there occure build up of electron on nylon surface and so removing nylon cloth electron are discharged and they cause cracking 

Answer 2

There are static charges developed when the nylon clothing rubs against other dry soft materials. like inner wear , or hair.  The static charges remain on the material as you know about the hair strands.  They stand up erect.

When these materials are touched or bent - the static charges get discharged - due to the combination of positive and negative charges - there is a crackling sound -  some electrical / heat (produced when charges come together) / mechanical (due to movement) energy is converted in to sound energy.

Even when hairs  on hand meet the television screen, we can hear the same crackling sounds, some times.

When one undresses, the materials rub against each other - so charges of opposite signs come together and fuse.  So energy is released in the form of light, spark, and sound.