it is not irrational belief
thee is a scientific logic behind it
the baby, who was in the the protection of cozy womb within the fluid, suddenly comes out of it and to this world.... the baby HAS TO cry.

that is the reason - doctors / nurses / midwives used to tap the baby to make him cry..

I dont want to scare you or your neighbour

Pls do crosscheck with the doctor.. and have a close watch on the baby if there is any developmental delays.