How did Charlemagne unwittingly unleash a new round of invasions into his realm before his death?

Charlemagne's extensive military campaigns and expansion of the Frankish empire led to the weakening of local defenses and the fragmentation of his realm. After his death in 814, his grandsons inherited a vast but unwieldy empire, struggling with internal divisions and succession disputes. This instability made the empire more vulnerable to external threats, particularly from Viking raiders, who began launching invasions along the coasts and rivers of Europe, exploiting the weakened state of Charlemagne's legacy. Thus, the very expansion and consolidation of power that Charlemagne achieved inadvertently set the stage for new waves of invasions.