Even as it's protected inside a thick skull, the brain remains a fragile organ. When we bump our heads or otherwise sustain a moderate to severe head injury, the brain gets sloshed inside the skull. This "sloshing" motion is the brain being banged and bruised against the skull, which could lead to a concussion if the trauma force is great enough.
In some cases, the concussion leads to amnesia - the inability to recall memories. Scientists think that as the brain gets jostled in the skull, fragile neuronal connections can get damaged. And this can extend to other regions beyond where the brain sustained the trauma. The trauma also triggers a shift in chemical responses, namely the neurotransmitter acetylcholine, which affects attention and motivation. Combined, researchers think the damaged neurons and the chemical imbalance contribute to memory loss following a concussion.