That can be from anything that increases the pressure in the brain such as swelling, bleeding or masses and is called herniation.
It can be from a birth defect known as a Chiari malformation.
The wedging of medulla oblongata into the foramen magnung is termed "incarceration".
This the result of pressure build-up in the brain.
This pressure build-up is typically caused by swelling of the brain, bleeding in the brain (as caused by a vessel-rupture, ie.: anurism) og by blocked drainage of the cerebrospinal fluid - often known as fluid in the brain or "hydrocephalus).
foramen magnum
medulla oblongata from the spinal cord
cerebrovascular accident
This is called a tonsillar hernation, where the cerebellum is pushed through the foramen magnum. The extra brain trying to pass through the foramen magnum can cause compression of the medulla, which has serious complications (e.g. stop breathing.) This may be due to a space occupying lesion somewhere else in the brain, or in the cerebellum itself. As the brain cannot expand outwards (due to the hard skull) it expands down (and across where possible). Examples of space occupying lesions include haemorrhage, oedema and tumours.
The obturator foramen is the largest foramen in the human body.
obturator foramen
Foramen magnum
The foramen magnum is where the spinal cord enters the cranial cavity.foramen magnum
The foramen magnum is located at the base of your skull.
foramen magnum
Removal of a portion of bone posterior to the foramen magnum in order to allow greater space for the nervous system structures within.
The spinal cord, proper, does not begin until past the foramen magnum, which is the exit hole in the occipital bone. The brain stem actually extends down to the level of cervical vertebrae numbers one and two. That is not only why people with a neck fracture die, but also they can live if the break is below the brain stem.