Why did Eli fall?
Radak: It was amidst great pain over the capture of the Aron.
What is "Yad ha'Sha'ar"?
Radak citing Targum Yonasan: It is opposite the path to the gate.
Malbim (4): It is the high point of the city.
What is "Mafrakto"?
Rashi: It is the neck bone. Radak - it is called so because it is made Perakim Perakim (of many parts).
Why did Mafrakto (his neck bone) break due to the fall?
Malbim: (a) He fell from a height (the chair). (b) He fell backwards. (c) He fell from Yad ha'Sha'ar, the high point of the city.
What is the meaning of "v'Chaved"?
Radak #1: He moved slowly, due to old age.
Radak #2: He was heavy. He died from the fall due to his weight.
Why does it mention here that he judged Yisrael for 40 years?
Malbim: This is one of three reasons why he died immediately - his strength waned from great work. The others are (a) He was old, and (b) His body was heavy.