Overtaken by another rider intent on wiping us both out...

[nickyboy]

Or another possible scenario:
- Car approaches the two cyclists and moves out for a wide, safe overtake. Cyclists are one behind the other, in-line and close together since second cyclist has failed to assume a primary or overtaking position in good time.
- Following cyclist, now very close behind OP, swings out for quick overtake (either noting that the car is a long way out or not taking any notice at all)
- Car is still laterally clear of both cyclists, since he/she was doing a nice, wide overtake, even two a breast as they are now, and already effectively committed to either the overtake or performing a sudden braking, but is alarmed by the second cyclist's sudden overtake and hits horn in annoyance.

Without the OP knowing what was happening behind, there really isn't a definitive answer to who was in the wrong and by how much.

We weren't there but I suspect this is pretty close to the mark.

When I catch another cyclist and prepare to overtake I always look behind to see if there's any upcoming traffic. If there is then I wait until it's passed. If the overtaking cyclist doesn't look behind (or even if he does) and then pulls out to overtake at the same time a car driver is overtaking, it must be pretty scary for the car driver