The answer to 'why is my rat infestation' is that rats may have infested your home due to a variety of reasons, such as seeking food, water, or shelter. Common signs of a rat infestation include droppings, gnaw marks, and hearing noises in the walls or attic. To resolve a rat problem, you'll need to identify and eliminate the source of the infestation, which may involve sealing entry points, removing food sources, and using traps or baits.
If the infestation is severe or you're unsure of the best approach, it's best to call a professional pest control technician. They have the expertise and equipment to effectively and safely remove the rats and prevent future infestations.