What were the greatest threats to rural life in the eighteenth century?
Some of this depends on the country you are discussing. In England, the Enclosure (sometimes spelled Inclosure) Acts significantly reduced the common land available to the peasants for grazing animals or growing crops. In Scotland, the Highland Clearances displaced small farmers to make way for large scale grazing operations.
The greatest factor affecting rural life and leading to depopulation of many rural areas was the Industrial Revolution. Starting slightly earlier in Britain than the rest of Europe, it represented a vast creation of wealth outside the agricultural sector. Many of the poor flocked to the cities in search of new jobs in the factories.
In part an outgrowth of the Industrial Revolution, the British Agricultural Revolution meant improved efficiency in farming, which had two effects. First, it especially improved the output and desirability of large estates, leading to consolidation. Second, it reduced the need for labor, adding a push factor to the pattern of urban migration motivated by the availability of urban jobs.
The combination of geographical mobility and the increasing growth of Protestantism also destabilized the parish system which had been the major organizing structure for poor relief and keeping records of births, marriages, and deaths in the immediate past.