A fight occurred and the victim fell on the stairs and later died.
Personal Injury Lawyer
Perhaps the probable cause supporting the charging document indicates that the fight was the cause of the fall, and the death was a direct result. Under such circumstances it would not be abnormal for the District Attorney to charge 1st degree murder. Of course many evidentiary questions come to mind, e.g. what was the cause of the fight, etc. At trial, the defendant would be provided an opportunity to challenge the charge and perhaps argue self defense if the facts support it.