Fighting Wood Pigeons

Hello.  We feed all kinds of birds in our garden and usually they wait for feeding times and get on with the business of eating. We do occasionally get Starlings squabbling a bit, but this year we have two Wood Pigeons fighting. This has been happening for a few weeks now and it's almost certainly the same two birds. They land on a small feeding platform, eat seeds and grain for a short while, then they square up to each other, puffing their chests out and stretching to their full height. They rarely attempt to peck each other but advance and retreat backwards and forwards, slapping each other with their wings, mostly taking turns, but it sometimes develops into a free-for-all. Yesterday, they both ended up on the lawn and continued the fight until they both suddenly stopped and started feeding - fight over. This bout of aggravation lasted over 15 mins. I got it on video as I was videoing the whole bird feeding scene.  Has anybody got any idea why this has almost developed into a ritual, happening daily (as far as I know), and why they fight when there's loads of food?. Maybe they fight in other places too, I don't know.