Joe is correct, it is due to the high density of glutamate sensitive NMDA
receptors within the CA1. During hypoxic episodes, massive amounts of
glutamate is released which binds to the NMDA receptors, allows more Ca in
etc etc into a downhill self-perpetuating spiral to cell death. For a
good review see the "god" of excitotoxicity - Choi, 1988 (I think that is
the correct year).