Too much stuff. The skull is a tight box. The contents maintain a constant pressure most of the time, and all goes well. If there is too much stuff that develops, the pressure inside the box may increase. Sudden excess blood, excess brain water, excess brain tissue, tumor tissue, can all result in increased intracranial pressure. Brain swelling can occur from many causes.
Many. Like tumor, meningitis, bleeding, venous sinus thrombosis, pseudo tumor cerebrum etc to mention a few.