Why does light diffraction happen? Does it relate to particle-wave duality?

Diffraction is a fundamental interaction of light with matter. Specifically, photons interacting with electrons. Photons are oscillations of the electromagnetic field, which electrons are particle manifestations. Diffrraction, refraction, and reflection are all just interaction of photons with electrons. Usually, the electrons are in orbit around protons bound to neutrons. Much of our existence is saturated with this form of matter, electron-proton-neutron type elements and compounds that is. So photons will behave accordingly, interacting via the energy levels quantized per electron orbitals and per corresponding photon frequency.

The particle-wave duality is just an illusion of detector imprecision. In other words, we cannot see the dimension it which it flows; let alone if it spreads or not.