Ah, sweet closure. I'm now sure that the problem was with the leader of the squad.
I've started a new fortress. When the greenskins arrived to siege us in 1554, I noticed one of my squads was loitering around again. As it turns out, the Axe Lord leader of the squad was loafing about with half a plump helmet roast in his mouth. The members of his squad were standing about nervously watching him eat.
"Didn't you hear the call to arms?" I asked him somewhat sarcastically. Surely he was as aware as I that the Gods had not yet seen fit to implement deafness. "We're under siege!"
The perfectly agile oaf opened his mouth and spit little roast bits out as he answered:
"I wuff hungee. We'll kilf the goblinf <chew> whenf I'm done eafing."
I pointed at an Axedwarf wearing the same squad colors as this oaf. "YOU! You're now in charge. Give the squad whatever Armok-be-damned name you want to. But get yourself up to the gates or I'll assign the whole gnomeblight-drinking lot of you to fertilizer duty when the river thaws out next summer!"
The squad, newly renamed 'The hotpants of defecation' or somesuch nonsense, hustled up to the gates. I would have stayed around to berate the demoted Axe Lord, but I didn't want to yell at a dwarf who had spent the past three years doing nothing but spar in the barracks and cut open the occasional goblin scout.
Thank you Hyperturtle. Excellent suggestion. Also thanks to Derakon: I'm now ordering walls and fortifications built far beyond the entrance of the mountainhomes.