The explosion of the Scandinavian people who went a-Viking was caused by a fast increase of population due to improved climatic conditions (which is similar to what happened when the Arabs boiled out of Arabia).
Actually, it's regularly disproven : there's no trace of a demographical "explosion" in Scandinavia, more clues of a situation in the continuity of previous centuries.
Migrations from Scandinavia were relativly rare, and only happened later : the first raids were only hits-and-run operations.
In fact, even later settlement were essentially just for creating winter quarters close to targets, preventing the trouble to go back to Scandinavia and were fairly limited in numbers.
Even the settlements as in Iceland or Groenland were more based on inner troubles (escaping vandetta, by exemple) than a real need for lands.
The first motivation of Vikings raids are the disruption of North Sea trade from a part, after the collapse of Frisians that dominated it, that forced Scandinavian to obtain goods from "source"; and the new impetus for trade that was Arabo-Islamic world that only increased the long range trade tendency that existed since the VIth century.
Not that other reasons couldn't have played as well : Frankish pressure (on Frisians, but as well on Jutland while it doesn't explain why first raids occured in England rather than Francia), collapse of Carolingia and economical crisis of Arabo-Islamic world, and possibly a limited need of settlement (but again, noticable settlements only happened at the end of IX century and up to the middle of X)