I think mostly to do with motorists believing they have the right and priority on roads.I think attitudes vary massively within the UK too, I have noticed a difference between large towns and more rural areas. My daily commute sees me riding from home through a village and small town on way to the station, and through central London on the other side, and return in the evening. Five days a week for four years I have had no abuse, encountered no road rage or anything like the attitudes displayed here from any motorists. One minor accident (not my fault) which can be put down to how busy the roads are and the driver was most apologetic. But there cycling is so common I think it is just seen as just another mode of transport and just as valid as any other.
If only I could say the same for my home area. I get abuse shouted at me from passing cars a couple of times a week, occasional deliberate close passes and on a few occasion having been forcibly pushed into the kerb. This sort of thing happens even at 6:00am when there are only a handful of cars on the road and people are able to pass easily without any delay. The only conclusion I can come to is that there is a much higher proportion small minded knuckle draggers in rural areas.