One of the primary causes for the rise in the number of smaller land holders was a decline in the custom of primogeniture. Towards the end of the Kamakura period, inheritance began to be split among a lord's sons, making each heir's holdings, and thus their power, smaller.
Over time, many of these smaller fiefs came to be dominated by the Shugo, Constables who were administrators appointed by the shogunate to oversee the provinces. Resentful and mistrustful of the interference of government officials, they banded together into leagues called ikki. The uprisings that resulted, particularly when the Shugo tried to seize control of entire provinces, were also called ikki; some of the largest and most famous took place in Wakasa Province in the 1350s.