Do we know why the maya civilization declined

The correct answer is - Other tribal units that were eventually assimilated were Indo Aryan, Xianbei, Hunyu, Scythians, and Han Chinese.

The Xiongnu people are a people of lot of mysteries. They did had a very large empire for around three and a half centuries in the eastern part of the Eurasian steppe, but they were not very fond of writing and living documents to witness for them in the future.

The only sources about them are Chinese, and even the Chinese did not had a lot of information about since the Xiongnu were very hostile and did not allowed such a thing.

They are considered to be a confederation of tribes that merged and became a large empire, but the problem is that it is not certain which tribes were those, and what type of people were they. What is certain is that the Scythians were not one of them as they were further on the west from them, there were no Indo-Aryans as well as they did not lived in that territory, and there were no Han Chinese as they were in the borders of China (eastern half of China nowadays), so we can exclude the possibility of these tribes being part of their empire.