Why did the borders of the Frankish state change repeatedly?

The borders of the Frankish state were constantly changing due to instability within the state and external factors. Until the 8th century, the Merovingian dynasty ruled there, which either annexed territories, then lost them in the fight against nomads from the east. During the reign of Charlemagne, the borders were established officially and began to be protected. Fortresses were erected there, fortifying borders and cities. After the reign of the king from the Carolingian dynasty, political fragmentation began in the Frankish state. The boundaries have changed again.