If we take a look in the triboelectric series, we find that wool becomes
more electronegative when rubbed against human hair. When we wear
woolen clothes, the initial charge on the wool and body is zero. When we
take out the clothes, the wool gets rubbed against our skin and get
charged by friction. During this process, Q amount of negative charge is
gets transferred to the wool leaving our skin with a +Q charge. It is
this transfer of charges that cause the crackling sound.