To go more into detail, Edward the Confessor supposedly promised the throne to William (can't remember the date or source) and this was William claim to the English throne and reasoning for the invasion. I'm hardly educated on the subject so anything that can point out the main causes for the event is highly appreciated! (And if someone could verify Edward's promise that would also be great)
Allow me to refer you to my Battle of Hastings/Norman Conquest/William the Conqueror megathread.
We can't really verify any of the supposed causes of the war, other than to say there's an Anglo-Saxon narrative and a Norman narrative. In the Norman narrative, Edward promised the kingship to William, who was also one of Edward's closest living blood relatives (first cousins once removed). What's more, Harold Godwinson swore on holy relics to support William's claim to the English throne. The Saxon line is more Edward died, there was no heir, no one had been promised the throne, and we elected this guy, Harold Godwinson. Which you believe is largely a matter of opinion.