Why do amino acids stimulate glucagon secretion?

and to add to Peter's answer, glucagon stimulates gluconeogenesis by activating PKA ( via cAMP ) which phosphorylates PFK-2 inhibiting liver glycolysis and stimulating fructose 1,6 bisphosphatase which is rate limting enzyme for gluconeogenesis. The glycogenic amin acids are stripped of the amino group and the carbon skeleton is processed so as to produce intermediates that can be used in making glucose and perhaps ultimately storing the synthesized glucose ( depending on energy demand ) as glycogen ( insulin stimulates storage of glycogen ).