mutiple rabbits

hi everyone,
I am having real behaviour problems from my buns at the min. i currently have 6 buns (3 pairs) all male/female and all neutered. 2 of the pairs are kept outside in a shed/avairy set up connected to each other but borded up so they cant see each other. the other 2 have free reign off the dining room which is about 10ft. The problem is that recently when i let them out for their free range time (seperate pairs off course) they have started behaving aggressively towards each other, nipping and chasing which they never did before. I understand that the reason for this is prob the scent of the other bunnies on the grass but what can i do to stop the agression before it escalates into a broken bond. They all have plenty of space but i dont want to take away their binky time but i also really dont want a fight on my hands. They have been all been bonded for at least 6 months so i dont understand the sudden behavior. Any ideas guys?

It is partly the time of year too. It is what is known as spring fever, it is coming up to rabbit mating season and they tend to get a bit frisky, it should die down in a few months

I have quite a few pairs of rabbits and have never had any aggression due to others being near by, except for one pair where the male was a very stressy bun for a bad home. Main can see and sniff each other through the wires and there are no problems. Its probably the time of year.