What caused the citizens of the Native American town to build another layer on top of the mound?

The correct answer is:

The death of a chief.

It's believed that the different cultures that are collectively known as "Mound Builders" would crash one of the wood-and-mud constructions to bury the remains of a deceased chief in a fresh layer of soil, to then raise a new construction on top. Usually, theywere laid to rest in specially developed burial mounds, conical or round.

The death of the chief is what started it and illnesses in the town is what made them add more layers