Is it due to adolescent behaviour?

Question
I have 2 netherland dwarf rabbits, between 3-4 months of age, both female. They used to be fastidious about their toilet habits, peeing and pooing only in the cage with more than 80% of them in the pee tray. They have full access to a playpen 4m by 5m big, together with the cage. They have never pee-ed in the playpen, though there are some occasion droppings that flew out of the cage when they hop out.
Recently, things have changed. Droppings are neatly scattered all around the playpen and they pee randomly in the playpen.
Why the change?  

Answer
You could have your vet check them out, but it does sound like your bunnies have hit the teenage years.  Bunnies have a caste and territory system.  Some rabbits are less tied to the "rules" of hierarchy and society.  And other are very strongly tied.

Bunnies will use urine and their poops to mark the territory.  Bunny societies often have one rabbit as the alpha.  It sounds like your bunnies might be starting the process of determining who is going to be top rabbit.

Are your bunnies spayed?  If not, you might want to consider it.  Fixing rabbits does help reduce territorial behavior.

Here's a little bit more info:
http://forums.petfinder.com/viewtopic.php?p=1393646
http://www.paperglyphs.com/rabbits/rabbittalk.html