Soil brought in on dirty shoes?
If a site is abandoned, earthworms may be responsible for burying it.
The Grass and other vegetation builds up the soil level in open areas, particularly farmland.
In built up areas it is mostly human activity ,Trash , Dung, muddy shoes :) ,and even river silting, that causes it to rise.
It has to be remembered that the vast majority of the sites that we now consider as historically important and so maintain ,where for many centuries at the mercy of “Quarrying” and Vegetational Soil deposition.
The rate of burial is not constant through out the land, it is dependant on Soil type and local environmental factors, and hence the growth characteristics of the local Vegetation.
For example the concrete building footings of the facilities used by Barns Wallace in the New Forest to develop the Tall-Boy and Grand Slam bombs have almost vanished from view in the Moss and grass that has covered the site. The Door steps are already under an inch of soil. That’s just in 70 years.