Usually a loss in litterhabits that are behavioural can include bringing in a new bunny or stress. Neither of these apply.
However, my previous bunnies had litterbox issues due to age, so she may have picked up on that. She's never been perfect, but poop is easier to deal with than pee on the carpet.
I've loaded the living room up with litterboxes, gave her low entry boxes in case it's arthritis, etc. It would be great if it's behavioural. I'm just not convinced :P