Eli fell backward: This was possibly from shock or a heart attack when he heard the message. His deadly fall represents his family's loss of the priesthood and the end of the wrongdoing his sons started (1 Samuel 2:12–17).
Eli had judged Israel: Being a priest and a judge was possible at the same time (compare Exodus 18:13–26; Deuteronomy 17:8–13). Samuel took over as Israel’s judge and provided the leadership Eli did not (see 1 Samuel 7:12–17).
The phrase forty years connects Eli’s career to the timeline and themes of the book of Judges (compare Judges 3:11; 5:31; 8:28; 13:1).