Given the following context:

Another reason for the gradual decline in the development of Sicily's Baroque and building in general was that the money was running out. During the 17th century, the aristocracy had lived principally on their landed estates, tending and improving them, and as a result their income also increased. During the 18th century, the nobility gradually migrated towards the cities, in particular Palermo, to enjoy the social delights of the Viceroy's court and Catania. Their town palazzi grew in size and splendour, to the detriment of the abandoned estates, which were still expected to provide the revenue. The land agents left to run the estates over time became less efficient, or corrupt, often both. Consequently, aristocratic incomes fell. The aristocracy borrowed money using the estates as surety, until the value of the neglected estates fell below the money borrowed against them. Moreover, Sicily was by now as unstable politically as its nobility were financially. Ruled from Naples by the weak Ferdinand IV and his dominant wife, Sicily had declined to the point of no return long before 1798 and again in 1806 when the King was forced by the invading French to flee Naples to Sicily. The French were kept at bay from Sicily only by an expeditionary force of 17,000 British troops, and Sicily was now ruled by Britain in effect if not in name. King Ferdinand then in 1811 imposed the first taxes, at a single stroke alienating his aristocracy.
The tax was rescinded by the British in 1812, who then imposed a British style constitution on the island. One legal innovation of this time of particular consequence for the aristocracy was that creditors, who had previously only been able to enforce repayments of the interest on a loan or mortgage, could now seize property. Property began to change hands in smaller parcels at auctions, and consequently a land-owning bourgeoisie immediately began to flourish. Revolts against the Bourbons in 1821, and 1848 divided the nobility, and liberalism was in the air. These factors coupled with...

answer the following question:

What did the land agents do that caused the aristocratic income to fall? 

became less efficient