Q- Is there any narration which mentions that one should shave one’s head after one’s parents or close relative has passed away? If no, is there any reason why people act according to this?
There is no such practice in Islam!
Rather, this is a pagan Arab custom, that Rasulullah (sallallahu’alayhi wasallam) strongly prohibited.Sayyiduna Abu Musa Al-Ash’ary (radiyallahu’anhu) reports that Nabi (sallallahu’alayhi wasallam) said:‘I absolve myself/I have nothing to do with, those who: shout out while crying, shave their hair and tear open their shirt collars.(Sahih Bukhari, Hadith: 1296 and Sahih Muslim, Hadith: 283-285)
This is considered a severe warning from Nabi (sallallahu’alayhi wasallam).
And Allah Ta’ala Knows best,
https://hadithanswers.com/shaving-the-head-in-response-to-calamity/