Why would Magnesium cause seizures? It terminates or reduces the frequency of seizures in pre-eclampsia and epilepsy.

http://www.ncbi.nlm.nih.gov/pubmed/22547512

http://www.ncbi.nlm.nih.gov/pubmed/21069663

Magnesium ions physiologically block ion channels associated with the N-methyl-D-aspartate (NMDA) receptor, and can act to decrease seizure activity much like other NDMA antagonists.   However, hypermagnesia can act as a vasodilator.  Anti-angiogenics like Avastin can create permeability fluctuations and sudden changes in hemodynamics and/or extracellular fluid dynamics in enclosed spaces 

(such as may be present in micro-environments of per-BBB regions of tumor) may lead to sudden shifts in either fluid and/or electrolytes.  So the magnesium itself is not causing the seizures per se, but setting up unstable regions which may lead to  seizures in people already susceptible to seizure due to necrotic metabolites.

I have a patient who experienced increased seizures after taking 800 mg of magnesium while taking Avastin. for treatment of a brain tumor.   He was also on keppra, a different AED.   If the magnesium added an additional level of anti-seizure measure there is no way he should have had 6 seizures in one day after taking the 800 mg of magnesium.  So "something" happened!   I understand Mg is used to treat eclampsia and hypertension, but no one has stated this happening in conjunction with antiangiogenetics.   That is my primary question.   Simply telling me that magnesium helps with "some seizures" is like saying drugs used off-label will NEVER have abnormal effects.

Apologies, your original question was framed as an abstract concept, not a specific event. 

Magnesium therapy increases placental VEGF receptor expression in pre-eclampsia. So perhaps it also enhances VEGF receptor expression in neural tissue. It may not be that the Avastin potentiated the Magnesium, but rather that the magnesium potentiated the VEGF inhibitor and it was this enhanced effect upon the tumour that caused the seizures.

Or perhaps neither is directly causal in this instance.

http://www.ncbi.nlm.nih.gov/pubmed/23725083

Before play chess you should known the pieces, and I probably help in the basic.

Magnesium ion has an ionized and intracellular fractions, being the 2nd largest intracellular cations in our body, critically important with potassium ions in most cell functions, as enzymes, at times being part of enzymes or protein structures. In fact, RBC magnesium is about 3 times of the plasma fraction. 

In blood, similar to calcium, magnesium is found as bounded, ionized and intracellular fractions (RBC), in which bounded or Mg salt represents the reservoir to maintain ionized Mg, the only functional fraction, except when bounded to enzyme or proteins.

In pharmaceutical form, we usually refers as 100 mg of magnesium oxide or 600 mg of magnesium sulfate. Unfortunately, in 500 mg of Mg sulfate there is only 5 meq (10 mg) of Mg. In other ions as calcium: 1 g of IV calcium gluconate (FDA: 8 mg of calcium), very effective but tiny fraction as compared with calcium chloride.

Therefore, taking 800 mg of Mg sulfate = 6.4 mg (3.2 meq), half of RBC Mg/liter cell, is harmless from pharmacology and physiology aspects. However, 793 mg of oral sulfate after absorption may decrease blood ionized ions, thus cell function, while 793 mg of IV sulfate rapidly bounds ionized calcium leading to hypotension, seizures and bleeding in presence of surgery. 

In brief, don't worry about our metal ions, our body has a lot, take care of the ionized and intracellular fractions (iron in Hb) which maintain us alive!