These over-sized rodents are the result of ambient radiationmutating common rats.
Having grown larger from their mutations, rad-rats have the aggressiveness that goes with it. Instead of scurrying away from anything larger than them, rad-rats attack with reckless abandon. They live in small nests, so when one rat attacks, they all attack, biting with their chisel-like incisors.
Rad-rats are not especially challenging creatures, but the fact that they are typically encountered in nests of 3-5 rats, move fast, and are surprisingly resilient means they can surround and overwhelm an unaware victim. Despite similarities to mole rats, rad-rats are much stronger than them, with the weaker ones comparable to a a mole rat brood mother.