In the Rogue One film, during the fight scene between Chirrut Îmwe and the stormtroopers at Jedha, Chirrut kicks up some sand towards a stormtrooper, the stormtrooper flinches and as a result gets knocked down a moment later.
Why did this stormtrooper flinch from a tiny bit of sand/dust while he was wearing his helmet? Was he scared and was this his reaction to possibly being hit with something?
I dare say that it was just pure and utter defensive instinct. You can observe it time and again also in real life. Whenever something comes at you with high velocity your instincts and reflexes kick in when you become aware of it. And if you are not trained otherwise the first reflex is to protect yourself, which is just what the stormtrooper tries there...despite having a helmet. So it is a purely human reaction there.
On another note...a combat droid would have fared better there (example B1s) as they usually don't have that self preservation instinct that leads to one trying to shield oneself from harm and damage.
