Why do some cyclists jump red lights

There is no 'reason'. They do it for the same excuse that prompts motorists to speed, use cellphones while driving, run red lights, roll through stop signs - because they can do so with very little chance of being cited by the police.

Next time it happens, 'accidentally' swing your arm up to meet the cyclist right in the face - that usually stops them. You can always claim that you were just pointing something out to someone, or stretching, etc. Sometimes the only way to teach some folks is by administering a sharp slap across the head.

By the way, I'm a cyclist myself and I hate seeing cyclists flout the law. I wish the police would come down hard on all traffic law violators, but if they won't do it for motorists who routinely and blatantly speed, what chance do we have that they'll do it to cyclists, who are far less common on the roads. Besides, police are so clueless with regard to cycling laws that when they do arrest cyclists, they charge them with stuff that is not illegal - for instance 'failure to use a bike lane', which has never been illegal in the UK.

@ Matthew Cream - that excuse does not wash when it's a pedestrian crossing, which is triggered ONLY if a pedestrian has pressed the button and has nothing to do with the metal detectors in the road.