How did the Kingdom of Kongo Decline?

Mainly due to interference from their Portuguese trading partners who later colonized them.

The Portuguese traders were greedy. They wanted to control the kingdom's copper mines and control the trade of goods from Kongo.
They also wanted to impose their own laws, and convert the people to Christianity.

To make matters worse, they also kidnapped the indigenous people and took them away as captives.

This resulted in great friction and continued conflict between the Kings of Kongo and the Portuguese.

Due to the conflict and ensuing instability, The Kingdom of Kongo went into decline around the 16th century CE.
It was  eventually absorbed as a Portuguese colony in the early 20th century C