Concussion
During an impact, the skull accelerates rapidly. The brain, which has considerable inertia and is surrounded by cerebrospinal fluid, accelerates slightly less quickly than the skull and deforms under forces from the various membranes, blood vessels, and connective tissue that attach it to the skull. This rapid deformation, or strain, of brain tissue stretches and injures delicate nerve fibers, degrading brain function.
It may be easier to get it if you already was exposed to head impacts ( source).
Hits to the head can cause brain vibration ( source)
Fun fact: a strong impact doesn’t necessarily lead to a concussion:
In fact, researchers have long been puzzled by the lack of correlation between peak acceleration and injury. This study illustrates why such a relationship may not actually exist. ( source)
Related: Brain MRI