According to Blockman, Wim. "Urbanisation in the European Middle Ages, Phases of openness and inclusion", Living in the City, Urban Institutions in the Low Countries, Willems, New York, Routledge, 2011.
Bolded emphases mine:
"The relative overpopulation of the countryside, in the sense that there was a surplus of labour, was dealt with in a variety of ways. The existing social relations came under pressure, particularly the exploitation of farmers in the form of servitude with all its limitations on freedom and performance obligations. If the landlords wanted to maintain their position, they had to tolerate the fact that surplus workers were no longer tied to their farm, but could leave. Where they went depended on the possibilities that the environment had to offer. The most obvious choice was to cultivate the as yet unexploited landscape in the immediate vicinity, by clearing forests and draining land. In anticipation of higher yields, large land owners, such as abbeys, went so far as to offer ‘settlers’ favourable business terms for the uncultivated land. The main provisions had to do with personal freedom and lower duties, which were of particular importance in the development phase. As the availability of land in the vicinity dried up, then people moved further afield in order to cultivate land." (Blockman 1)
"A third possibility to escape overcrowding was the development of another economic activity, for example in fishing or the crafts. Maritime work supposed shipbuilding, which in turn required a multitude of small businesses. A large part of this remained tied to agricultural production such as forestry, sail making and rope making. Other resources, such as logs that were suitable for masts, nails and tar and pitch, had to be obtained from coastal areas via long distance trade. Special forms of craft and trade activities were effectively exercised by the concentration of housing and jobs which were clearly separated, but never disconnected, from the countryside. This particular reception of the overpopulation from the surrounding areas marks the origins of the first spontaneous growth of cities. Ultimately, because the migration to the cities entailed not only different economic activity but also a profoundly different way of life it was the least obvious but the most radical solution to the overpopulation." (Blockman 2)
"The long growth phase from the tenth to the thirteenth century was followed by a period of roughly one and a half centuries, from 1300 to 1450, during which a considerable reduction occurred in the total, but also in the urban, population. Without doubt, strong migration from the countryside to the cities was based on the phases of growth, in both the period until 1300 and again after 1450. But it is also highly probable that the outflow from the countryside persisted during the long period of depression from 1350 to 1450, although regional differences can be identified. The direction of the migration also changed, following a varying assessment of the unfolding opportunities. As a result, the explanation for the migration to the city demands a different answer for each of these three periods. The first phase is characterized by the formation and growth of cities out of small old centres by foundations that appeared from nowhere. During the later phases urban institutionalization had already occurred, which may have exerted its own pulling-power. This could include the institutional social services which built up over time in the cities. It remains, then, to explain the causes of these significant regional differences and why migration persisted, both during times of growth and times of decline." (Blockman 2)
"Environmental historians have put yet more fundamental aspects of urbanization on the agenda. They point to the vital link between a city and its surrounding areas. In the most general sense, the constant supply of potable water, food, energy and materials is essential to a concentrated urban population; at the same time, in order to keep the environment viable, they must be able to eliminate waste. In biological terms, a permanent and relatively large population concentration is only possible if the surrounding area can be intensively exploited. Thus, the interaction between urban and rural stimulates the intensification and diversification of agricultural production. An urbanized society brings about a transformation in the ecosystem, both in the countryside and in the city. In such a society, the space, matter, energy, the exchange of information and the allocation of time are designed in a new and particular way. In biological terms, the urban way of life offers the human species the information processes that make it possible to multiply and to accelerate. The size, density and diversity of an urban population increase the effectiveness of sexual contacts and, therefore, reproduction. This boosts cultural performance, leading to an increased exploitation of natural resources and manpower." (Blockman 2)
"Also in the period of a strong overall decrease in population, from around 1348, people continued to move away from the countryside. They settled in cities, despite the chances of dying clearly being greater. " (Blockman 10)
While the increase in population in the early Middle Ages did increase urbanization, urbanization continued through the the High and Late Middle Ages and the period of decreased population, due to new pulling power of urban institutions and growing links between rural and urban economies. So I was partially wrong before.