What triggers the release of glucagon?

Low level of glucose in blood, adrenaline or high protein meal.

Explanation:

Glucagon is secreted by alpha cells of pancreas. Glucagon and Insulin work together to maintain the blood glucose level.

Glucagon is released in response to low insulin level alternatively low glucose level in blood.

Glucagon increases the blood glucose level by activating the gluconeogenesis (Synthesis of glucose from amino acids etc) and glycogenolysis (Conversion of glycogen to glucose).