You're right that it probably is due to hyperkalemia- although it's probably due to a sudden transient release of potassium from damaged muscle cells into the bloodstream as a result of the seizure, rather the absorption of K+ into cells. The other possibility is that the T wave alterations are a sign of myocardial ischemia brought about by hypoxia and/or catecholamine release during the seizure episode. Without lab tests and a 12-lead there really is no way to be certain, but one of the above situations would be my guess.