One possible reason why Harold was defeated at Hastings in 1066 was that he had a lot on his plate.
One would have thought that fighting an invader on home turf would have given home-court advantage to the English. Also, the fact that the invasion came in by sea – and by sail – should have given Harold some time to prepare. Sailing vessels with heavily armed soldiers are not exactly stealth bombers. There were, however, other problems.
Harold was busy in York putting down a series of insurrections and invasions by King Harald of Norway. The Norse landed in Scotland, found allies there, drove their way into England and on 20 September 1066 fought a pitched battle just outside of York – Fulford Gate. They then proceeded to burn the city of Scarborough to the ground.
on 25 September, King Harold (not Harald) arrived from London, put up a good fight, and felled Harald (not Harold).
Meanwhile, back at the ranch, the Normans land unopposed (everyone else is busy elsewhere) on 28 September. On 2 October, Harold (not Harald who is dead) left York and headed for London. They arrived in Hastings on 13 October, which in itself is a remarkable feat.