Found this on the 'net:
Short haircuts became a tradition after campaigns were fought in tropical climates - the danger of scalp infestation was greatly increased. Coupled with a general advance in medicine, the armies were led to require their field soldiers to cut their hair very close to the scalp in order to make it easier to clean, make scalp wounds easier to treat, and to make hair less trouble to maintain in battlefield conditions.
Maybe skinheads shave their heads so that nobody can pull their hair!