chemical violence

Halogenation reactions are violent violent things. A molecule is just sitting there, and then a nucleophile comes by and it dives in from the back and slams into the molecule, popping off one of its other substituent groups (in this case, a halogen).  Like those physics balls that click forever, except for mostly the substituent group doesn't come back.

If it can't attack the back of the molecule, it runs by and rips off a hydrogen instead.

nucleophiles are jackasses.