Was manorialism more a cause or a result (or both) of the advent of the “New Agriculture”? How did its operation relate to feudalism? What human needs did manorialism meet?
Relation to Feudalism:
It appears that Feudalism and Manorialism are used interchangeably in some sources; the one below suggests that Manorialism evolved out of Feudalism. In the former, the lord owned everything; in the latter, the serfs had more of a stake in land ownership, in some cases. The "New Agriculture," with its innovations, happened during Manorialism; maybe due to the fact that the serfs had more of an economic interest in the land to make it produce, rather than being near-slaves who had no interest. See more at the link:
One factor to consider in addressing the cause or result question is that under the feudal agreements between the three classes of warrior nobility, cities were taxed more heavily. In some ways this taxation strangled cities (a feudal economic collapse of sorts) driving more people to farming as a means of subsistence (the opposite of what was to come later with the Industrial Revolution). From this initial move toward farming and toward the increasing build up of manorialism under the banner of feudal lords came the circumstances resulting in the development of "New Agriculture."
I would actually say this is one of those questions that is very difficult to answer directly. In a sense, manorialism was both a cause and a result, as it certainly was something that was impelled by "New Agriculture" but at the same time it was something that also acted to spur it on through the mutual need of farmers to have protection and big estates to have labour.
It was a necessary condition for the "new agriculture" as it secured the labor to farm on large manors. Much of its effectiveness can be attributed to the warm climate that preceded the "little Ice Age." Generally though, agreements that amounted to manorialism were formalized versions of pre-existing realities. They were also highly negotiated, and not necessarily as prescriptive as one might imagine. In terms of its relationship to feudalism, often manorial, or seigniorial rights would be transfered to lords who inherited, were granted, or received lands through marriage.
Manioralism arose out of the same sort of problems that led to feudalism. You had small farmers who couldn't protect themselves and needed someone to keep them safe as they farmed. They also needed someone to support them if the harvest was bad. So they made a deal in which they essentially gave up their freedom in order to be protected, provided with land to work, and to some degree provided for in bad times.
So I'd say maniorialism was more of a cause. It gave the peasants the security they needed to build mills and some of the resources they needed for making the new tools involved in the "New Agriculture."