It's not. Brain tumors can present with many problems, seizures being one. We are often taught that if a seizure is the first symptom a patient has that leads to the diagnosis of a brain tumor, that they will likely have a more favorable outcome. There are a multitude of "gliomas" - some are relatively benign and slow growing, and those are the ones in which seizure can be the first sign.
Brain irritation. A common presenting system of a glioma is a seizure. It is important to note there are other causes of seizures. The reason that a glioma is prone to a seizure is that it is an infiltrating tumor that frequently involves the cortex. It irritates the brain both by physical approximation and also by causing edema (extra fluid accumulation). The treatment is anti-seizure medications and surgery.
Brain damage. Gliomas are tumors of the brain. They arise from glial cells, which are part of the support structure of the brain. As the tumors enlarge, they press on and irritate brain cells. That can make the normal brain cells give off electical impulses, and if there is too much activity, involving too many cells, a seizure will occur.
Gliomas. Gliomas are tumors of the support cells for neurons. Neurons rely on these support cells for metabolism of the neurotransmitters. We do not fully understand the development of seizures in brain tumors, but it is likely that these seizures are caused by dysfunction of neurons due to the metabolic derangements of these tumors.