reason behind this practice is that Allah (swt), the Exalted, has prescribed five obligatory prayers for people, and thus, one takbir is designated for the deceased from each of these obligatory prayers.
Al-Kāfi #694“Abu ‘Abd Allah (a.s.), has said that the Messenger of Allah has said, ‘Allah, most Blessed, most High, made five Salah (prayer) obligatory,...
ʿUyūn akhbār al-Riḍā #372On the surface it seems that this is because it has been taken from the five daily prayers. However, there is another reason for that....