Hey humans, let me poke your brain.
I found this on Helena Pierce's wiki. Its one of the unused audio logs that they could have had for the first borderlands game.
So, @queerzubat and I were talking about the differences between bl3 rakk and the rakk in previous games. We both seemed to have come to the conclusion that the reason for their differences (aside from just design choices, I'm here for the sake of science and that's a boring answer) is that they could have adapted to different environments; that bl3 rakk are inland rakk and the others are coastal rakk.
Now this line tells us that the east coast rakk are moving from somewhere, it's not specified, and they're choosing to stay in the rust commons area. This could mean a few things.
Animals tend to move from place to place for a few reasons, they're either migrating, being chased out, they're following prey, or someone brought in an animal from another place (if there are more please let me know, i'm not an expert).
The rakk found on the east coast are not migrating, unless Pierce is mistaken in that they are, it's just they've chosen the east coast sector as their stopping point. This could imply that their usual stopping point is no longer suitable for their needs, which could tie into the theory that they're being chased out of their usual habitat.
If the rakk are being chased out, there's a bunch of reasons for that too. Shrinking habitat, climate change, lack of prey (which is related to chasing prey), humans being dicks, or a stronger/invasive creature has moved in.
bl3 rakk - or inland rakk are extremely different from their predecessors, most notably in their heads. Their jaws look like they're built for grasping at prey, and the little toothy hole thing could be used to puncture the flesh and suck out their prey's blood.
Meanwhile, bl1 and bl2 rakk have flattened mouths and rounded jaws at the bottom, almost like a superior type fish mouth. Since they live on the coast, it could be that they use this type of mouth to scoop up small fish or sea creatures in the ocean and swallow them whole. They also got these little beard things on their mouths, which could also drain out the sea water so they're not gulping all that salt water along with their prey.
Also as far as I can tell, bl3 rakk (or bl2 rakk for that matter) don't have a relationship with rakk hives, even going as far as making their own nests. I don't even see them mentioned in bl3 either (they could be mentioned by characters, but I mean they're not mentioned when I looked up the maps for pandora in bl3.)
The only games where you can interact at all with a rakk hive are in Borderlands 1 and the Tales from the Borderlands (which some chapters are implied to take place somewhere on the east coast since it shares New Haven as a location).
Sadly, we don't know a lot about the migratory and hibernation habits of rakk hives, because I don't know enough about how the seasons might work on Pandora. That's going to involve imaginary plant hibernation habits and how the fuck the planet rotation works.
It's not explicitly said when exactly borderlands 1 takes place; however, I feel like it is implied that it's the beginning of spring thanks to the still dormant trees and the flowers being in bloom.
It could also just as easily be mid summer or start of fall. All I know is that the planet is in a transitional period, so the rakk hives are either going in or coming out of hibernation at this time (which explains the one rakk hive you have to fight in trash coast).
That being said, they're not seen in borderlands 2 at all, which means that most likely, they're solely found in the east coast.
It could be that the east coast rakk (rakk in bl1) are specifically a parasitic type rakk, following only the rakk hives.
Unless I'm completely wrong and they show up in bl3 which will throw this theory completely out the window.
I'm starting to feel like that guy with the wall full of sticky notes so I'm gonna stop here.