Really weird theorycrafting time!
So, to quote a section from some of the Lore Books findable in the game:
According to a book written by Rodrick the Roach Scholar...What we call the "Day of Awakening", the Roaches called the "Day of Reckoning".It was an event where they "woke up to the world."They share our belief that before that day, we were not different from the lesser bugs amongst us.A cataclysm happened suddenly, which banished the once fabled Giants to another world. Only bugs and some creatures were left in this world.After said cataclysm, the roaches became sentient, giving birth to civilisation. Other books by Rodrick mention fires raining down from the sky and destroying the earth.
I predict that the "fires" and the "Day of Reckoning" were an atomic bomb that mutated the bugs, causing them to become sentient. The "Fruity Bugs" may have decreased intelligence due to being farther from the blast. The Giants fled the land due to an incoming bomb, but one Giant (or maybe a house cat) failed to evacuate in time and mutated into Dead Lander Omega. Some household pests, (worms, spiders and flies, for each Dead Lander type) were mutated into the horrid Dead Landers.
Weird, I know.