If it was an isolated incident, it could just be he didn't make it to the litter box in time or got distracted. If he is peeing everywhere and it's a consistent problem, I'm wondering if your bun might be having bladder sludge issues, especially if he is starting to get urine staining around his parts, possibly from urine dribbling. You won't always see signs of it in a urine puddle with the urine being thick or gritty, as the calcium sediment can build up and remain in the bladder at times and not be expelled in the urine.
So if this is what's going on, it could be the kale you are feeding(and any other high calcium foods) is contributing to the problem. If this is what's going on and your bun is still eating and drinking normally, it may be possible to correct the problem by just altering the diet to lower calcium foods. Something that can also help reduce bladder sludge issues is to make sure bun gets plenty of exercise running around and hopping up on things(helps churn up the bladder contents so the sediment can be expelled easier). A low calcium diet would be a low calcium pellet, grass hay(low in calcium), no alfalfa, and low calcium veggies/greens. Lots of fluids and making sure bun stays well hydrated helps keep the bladder flushed out. I know of some people that have even had to switch to bottled water as their tap water was too high in calcium. Even though your bun didn't used to have a problem, some rabbits can develop these issues as they get older.
If he continues to urinate everywhere, is dribbling urine, and/or his eating or drinking is affected, then you'll need to get him seen by a rabbit vet right away as he may need his bladder flushed of sediment, and he could have also developed a UTI that would need the proper antibiotic to treat.
http://www.medirabbit.com/EN/Uro_gen_diseases/generalities/Sludge.htm (WARNING: this link contains photos of internal organs)
http://wildpro.twycrosszoo.org/S/00dis/Miscellaneous/HypercalciuriaRabbits.htm
https://rabbit.org/vet-listings/
