I think two of the things that piss off motorists are the failure of cyclists (particularly on road bikes) to stay left, and the tendency to not ride in single file. It gives the impression they're saying, "Fuck you - I'll ride where I like; it's MY road".
Kinda the same as the way many motorcyclists ride, I guess.

As for the "failing to keep left" thing, I'm guessing here, but I think it's largely because the edge of the road - particularly on country roads - is often in poor condition, not smooth (as it gets little traffic), and all the shit (stones, nails, car parts, rubbish) gets swept onto it by the traffic.
The riding two or more abreast thing may be understandable if they're in a race, but it kinda signals to other motorists, "I don't really give a toss about my safety". Doesn't being safe on the road start primarily with the rider? If they don't keep left, and ride in a bunch, they're relying on the other tards on the road to look out for them. And if one of the other tards happens to have to make a split-second decision between hitting one or more cyclists, or hitting an oncoming truck, it's a tough call.
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