Gross as it may be. it's worth pointing out that in Africa, scavengers such as vultures and jackals make short work of corpses. In other biomes, there might be fewer scavengers and so bodies may have been buried to prevent them rotting in the open and causing disease.

The addition of grave goods indicates that the lifestyle of the people was fairly prosperous, since they could afford to put items beyond use.

As to why they stopped, there could have been a change of custom, leading to sky burials or cremation (less likely), or a return to leaving the dead for the scavengers . Or an economic change that made the dead a valuable source of protein.

Special treatment of the dead is usually a sign of a semi-settled rather than fully-nomadic culture. A larger population in a given area tends to lead to a more sedentary lifestyle as a means of conflict avoidance. A reduction of population may have led to a return to nomadic ways for a time.

Ths is, of course, all pure speculation on my part!