So, so, so very close!
But it's actually a clear reference to the life of a plain "ugly thing" which is part of the pack that generated with a "very ugly thing". Since interpreting song lyrics is my speciality, allow me to explain.
Windows shattering is a reference to insults that ugly people were so atrocious that they could cause mirrors to break (
http://tvtropes.org/pmwiki/pmwiki.php/M ... ackingUgly). The regular ugly thing is clearly inferior* to the very ugly thing, which is typically much more powerful and a threat to player characters everywhere. By this time, the regular ugly thing is mostly filler that can be ignored, hence living in the shadow. Referring to the very ugly thing as a terrible thing is a double meaning; both because ugly things are generally considered bad (Damn you western society beauty standards), and also because of the very ugly thing's bullying and overshadowing of its band of regular ugly things. The alarm bells ring is likely the most poetic part of this - there are several metaphorical interpretations. One could be the alarm represents the danger to the player character that a pack with high speed and varied elemental attacks presents; another take is mocking the powerlessness of the ugly thing to have its voice heard over the much louder very ugly thing, who steals all of the attention the pack receives. There is of course the possibility of it being a literal alarm trap as you mentioned, but I consider this to be unlikely, it is too shallow and would be out of place surrounded by deeply symbolic lyrics.
Overall themes of powerless and oppression are common in crawl, with Pikel and Kirke being commonly noted examples. The subjugation of ugly things and the lower castes of orcish society are also rich topics worthy of further study. Because really, why would a single orc think that he should actually approach a level 19 player in the vaults, if not for fear of reprisal for retreating in the face of a battle he clearly knows he cannot meaningfully contribute to, let alone actually win?
In contrast to very ugly things/orc leaders, Yaktaur Captains and Centaur Warriors are excellent leaders who negotiate fair salaries with their respective unions and treat their workers with respect. The Centaur's contract in particular has an excellent dental plan.
*In terms of raw power, status in the dungeon society, and threat to adventurers. It is the opinion of this author that all lives have merit, and I do not wish to impunge any of the dungeon's denizens.