Who was the initiator of the split of Charlemagne’s state: his heirs or ordinary soldiers? Is it fair to call this split feudal fragmentation?

It was a split among three new peoples: the French, Germans and Burgundians. These peoples chose different descendants of Charlemagne as their leaders and began to build their states separately. Then the leaders did what most of the warriors wanted. Feudalism began to develop later, in the 10th century, when the warriors turned into landowners and became unequal in their rights.