I'm going to add a 'symbols' section to the page. I was originally going to attach a blurb to The Pack under the 'new enemies' section, but it got too bloated.
The Pack & Crawlers are an expansion of the consumerist theme that ran through the 1st game. Planet cracking is just a way of symbolizing one generation eating up all the resources and not leaving any for the next. So, the next needs to planet crack somewhere else, and so on. Well, these two enemies (The Pack specifically) show up in a mall area on the moon Titan. The mall being another symbol of consumption, and Titan being the first cracked thing; Titan was cracked because of a lack of resources, and that lead to an expansion of consumption.
That's it in a nutshell. Throw some other symbols at me, and I'll include them as I revise.