[With Reference to John P Cloherty, Manual of Neonatal Care, Fifth Edition,Pp547,Lippincott W&W]:

"For unexplained reasons even refractory seizures in HIE ultimately burn themselves out and cease after approximately 48 hours".

WHY DO THEY?

I don't know. One reason may be that seizures in HIE is originating in the inflammatory zone in the periphery of the injured zone, neurons that recover with time (or go in complete necrosis).

My experience is also that seizures after severe asphyxia can be very difficult to treat, and that they can persist for long times despite antiepileptic treatment.