Your body down-regulates how much gaba it needs to make naturally since when benzo's bind to the gaba receptor they enhance the potency/efficacy of natural gaba at doing its job. Less gaba is needed to inhibit while the benzos are acting, and when you discontinue use you are left with a gaba down-regulation that takes a while to return to normal levels. Gaba being the chief inhibitory neurotransmitter in the brain, when heavily down-regulated and abrubtly discontinued can lead to over-activity causing withdrawals, all the way from rebound anxiety to grand mal seizures, and possibly death.
from wiki-
from wiki-
http://en.wikipedia.org/wiki/Benzodiazepine_withdrawal_syndrome
Benzodiazepines potentiate the action of the neurotransmitter GABA. When this potentiation is sustained by long-term use, neuroadaptations occur which result in decreased GABA activity and increased excitability of the glutamate system. When benzodiazepines are stopped, these neuroadaptations are "unmasked", leading to excitability of the nervous system and the appearance of withdrawal symptoms. Increased glutamate excitatory activity during withdrawal is believed to result in kindling phenomena.[82] Those who have a prior history of withdrawing from benzodiazepines are found to be less likely to succeed the next time around.[83] Repeated benzodiazepines withdrawals, like with alcohol withdrawal, may lead to sensitization or kindling of the CNS, possibly leading to worsening cognition and symptomatology and making each subsequent withdrawal period worse.