Rabbits behaviour changes after change in hay?

Question
I have a 2 1/2 year old Netherland Dwarf rabbit. He is usually really lazy and just lazes around, even if let out his cage/playpen. He has been litter-trained since we got him, and rarely has he made 'mistakes'. However, his behaviour suddenly changed after we changed the hay he eats.
We used to feed him Timothy hay, but because there was no more stock at the pet shop, we decided to give him Orchard Grass Hay for the time being whilst we put the Timothy hay on order. Right after we did this, he suddenly became really active, drank lots more water and naturally, urinates much more. It's not a bad thing, to become active and/or drink more water. But, he also partially stops using his litterbox!
His cage is attached to a playpen (where the flooring is carpeted), and inside the cage, has a box of hay at one end and his litterbox at the other. I've heard and read that people say that rabbits like to eat whilst doing their business, but that has never been a problem for mine. He always eats and poops at the correct corners without mixing the two. However, after the change in hay, he suddenly keeps urinating in his hay box, and next to it as well! He still partially uses his litterbox, but most of the urine is usually found in his hay box/near it. This has been going on for a few days and I have to keep replacing his hay soaked in urine every 5 hours or so.

I've read that sudden changes in behaviour is considered bad, but this isn't necessarily a bad change. I'm only having trouble with the litterbox thing. Is there any reason as to why my rabbit has started doing this? I just cleaned his litterbox with disinfectant, in case it was getting too dirty (too strong smell) and he didn't want to use it anymore. I also cleaned the cage so that the urine smell wouldn't stay, hoping to encourage him to resume his business in his normal litterbox. I will check on his tomorrow morning as well, but just need some advice. thanks!

Answer
Dear Desiree,

Wow, that is really weird.  I can't explain it unless to hypothesize that the orchard grass has a scent that's making him crazy with the desire to mark it.  Is he neutered?  If not, then maybe the scent of wild rabbits who wandered through the hay is upsetting him and making him urinate on the hay to mark it.

That's really the only idea I have.  Has this happened with every batch of orchard grass hay?  Or is this your first try?  If the latter, then I'd be curious to know if the next batch elicits the same reaction.

Try timothy hay for the next time around, and then back to orchard grass.  If he changes back and forth, you'll know it's something about the orchard grass.  But at this point, you can't really be sure he's not having a behavioral change that just happened to coincide with the introduction of the grass.  So you need to rule that out with the little experiment.

Hope this helps.

Dana