They can certainly pick up things like mites and other parasites outdoors. But it is also thought buns could have a low population of some parasites at all times and when they’ve become a bit run down (maybe through stress, another illness or age) their immune system is not as effective at keeping parasites under control. I really believe that has occured with my rabbit Gooseman. He’s had furmite a couple times and he has some chronic health issues. He hasn’t been outdoors on the grass in a long time. Intiallly, I thought maybe he picked up the mites on one of his vet visits but I really couldn’t say. His bonded mate didn’t get anything like he did (though I gave her 1 dose or Revolution just in case) and the other pair in the room haven’t had mites. So it strengthens my suspicion about his immune system.
How’s Dasher weight been? Sometimes their weight can drop off a little when fighting parasitic or other infections.