That's a good question, and it can be answered many ways. I'd point you to:

• See most recently, /u/y_sengaku in response to Did the vikings begin raiding in the the 700s primarily because they were pushed away from home, or because they were pulled towards the rest of Europe? How would we guess?
• My own response to Was a change in culture or climate the single biggest catalyst for the Viking expansion?
• My own response to Did Medieval Europeans Know about Scandinavia?

I'm sure you'll find further responses in the FAQ or if you start searching around a bit more. For further reading, Neil Price's new Children of Ash and Elm (2020) will likely be the new standard on this topic. The first third of the book deals with this intensively, plus it's a great read. It significantly builds on the previous framework proposed by James Barrett in his article Rounding up the usual suspects: Causation and the Viking Age diaspora (2010).

You might also be interested in a recent AMA: We are AskHistorians flairs of the Viking Age! Ask us anything about Assassin's Creed: Valhalla!