I have combed through various sources on the internet and I don't have a definitive answer for the above question:
The best that I can come think is the following:
Because when I remove synthetic clothes, the body gets charged one way, the clothes the other way. Then as I continue the process of removing the piece of clothing off my body, the charge build up is significant, owing to which the potential difference between the cloth and my body is greater than the breakdown potential of the air. Thus, electrons flow between my body and the piece of clothing I am removing. The air heats up, gets ionized, energy is dissipated in the form of heat, light and sound; in other words, as sparks.
Also one more follow up question: Do the sparks flow "inwards" between my body and the piece of clothing? Or just dissipate "outwards" into the air, towards nothing in particular?
Is the above reasoning sound?
Also, does the same reason hold when I comb my hair? However, if this were true, why do I NOT SEE any sparks when I comb my hair but only hear a crackling sound?