How does being under the influence of both alcohol and marijuana cause seizures? Is having a seizure one way for the body to cope with the toxins?

See below. Marijuana is irrelevant. Alcohol withdrawal is associated with seizures secondary to its affects on gluamate and GABA tone in the brain. It has nothing to do with toxins (other than alcohol of course).
Multiple causes. Alcohol toxicity or alcohol withdramarijuana cause seizures marijuana may have certain toxins in it that might cause seizures as well.