The chronicler Paul the Deacon refers to Pepin being born ante legale connubium ("before legal marriage"), but does he mean Charlemagne and Himiltrude were eventually married? Or that perhaps they had a Germanic form of marriage bond (called Friedelehe, "lover marriage") that was less formal than what we now think of as marriage? Pope Stephen III in a letter to Charlemagne refers to him being married at a time when Himiltrude was the only female in his orbit. Himiltrude disappears from records in 770 when Charlemagne marries Desiderata of the Lombards, and then Hildegarde a year later.
Whatever the case, questions of legitimacy were raised about Pepin. About 781, on a visit to Italy, Charlemagne has another son, Carloman, by his wife Hildegarde, baptized by the Pope and rechristened "Pepin of Italy." This seems to signal that he was "replacing" the older Pepin.
Perhaps anger about being replaced built in the older Pepin, and in 792 he and a group of Frankish nobles rebelled against Charlemagne. 792 saw a famine after a poor harvest, and Charlemagne had been making some legal changes to consolidate his authority and prevent abuse in local courts. He also created a new loyalty oath and insisted that it be taken by all nobles. The Royal Frankish Annals (mentioned here) also cite the cruel Queen Fastrada as a reason to make changes at the top.
Pepin and the nobles planned a coup while Charlemagne was away in Bavaria. A Lombard learned of the plot and informed the king. (The informant was named Abbot at St. Denis for his loyalty.) The plotters had their lands confiscated, and some were executed. Pepin's life was spared, but he was tonsured and forced into a monastery. The Lorsch Annals state in 793 that, post-rebellion, Charlemagne lavishly rewarded all those nobles who were still loyal to him.
Pepin's monastery was Prūm, far from court. When Charlemagne decreed that, upon his death, his kingdom would be divided into three for his three "remaining" sons, it was clear that the still-living Pepin was being completely ignored.
The Royal Frankish Annals list his death as 8 July 810.
The musical Pippin is a highly fictionalized account of his life.
Those three "remaining" sons who would each inherit one-third of the kingdom were Charles the Younger, Pepin of Italy, and Louis. I'll tell you about them next time, and whether they managed to be satisfied with only one-third of the father's realm.