Howdy! Vikings were pirates, and although piracy was pretty common throughout the early medieval period, it seems to have taken on a new scope and social importance in northwest Europe between 800-1100 AD—the 'Viking Age'. Scholars have spent a lot of time arguing about the best and most useful way to define this period and the ways to think about the major changes involved. I'd point you especially to two brilliant articles by James Barrett, one describing the possible causes of the 'Viking Age' (technological development, environmental improvement, population pressure, etc.), and one describing the changes that we typically think of as the 'Viking Age' (state-formation, economic development, etc.).

• James Barrett, “Rounding Up the Usual Suspects: Causation and the Viking Age Diaspora,” in Atholl Anderson, James H. Barrett, and Katherine V. Boyle, eds., The Global Origins and Development of Seafaring (Cambridge: McDonald Institute for Archaeological Research, 2010), 289-302.

• James H. Barrett, “Maritime Societies and the Transformation of the Viking Age and Medieval World,” in James H. Barrett and Sarah Jane Gibbon, eds., Maritime Societies of the Viking and Medieval World, Society for Medieval Archaeology Monograph 37 (Leeds: Maney Publishing, 2015), 1–13.