There have been all sorts of theories. James Barrett has a review article in the latest Antiquity, and Archaeology in Europe has a summary. Barrett rejects that notion that the Viking expansion was driven by new technology; since Saxons raided across the North Sea in Roman times, whatever boats they had then were adequate to the task. He also rejects climate change. He ends up focusing on competition for wives among "surplus" men, and a sort of ideology of conquest and raiding:
The Scandinavian diaspora was not a product of technological, climatic or economic determinism. Nor did it result from 'overpopulation' or the lure of weak neighbours. Instead, bands of 'surplus' young men (perhaps resulting from selective female infanticide) in need of bride-wealth may have set out in search of treasure. As has long been recognised, they were joined by would-be chieftains, royal deputies and exiles - seeking wealth to prevail in the face of increasing competition within Scandinavia.Ingvar the Far-Traveled led 26 ships of Swedish mercenaries to the south coast of the Caspian Sea to fight in a civil war there; most died of disease, and only one ship returned. This was a crazy thing to do, but the deeds of Ingvar's men were remembered in a saga, and relatives of the fallen set up many rune stones in their memory. Twenty-six of these "Ingvar Stones" survive. Viking boys grew up hearing about the deeds of men who found glory across the sea, and many longed for their own voyages of adventure. The biggest threat to a Scandinavian king was that some exiled relation would return from overseas with a ship full of silver and an outsized reputation and toss him off his throne.