Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your situation, and wanted to help.
Again I do apologize that my colleagues could not aid you sooner and that Chase has passed away so abruptly. Now as I am sure you can appreciate, rabbits are a prey species. This means that they will go to great lengths to pretend all is well, even when they are not (since advertising you are unwell would make him a target for predation). Therefore, it is not uncommon for rabbits to see healthy one moment but in the advanced stages of deterioration the next. And this hardwired instinct means that while we do have to consider toxin/poison exposure, we also have to consider issues that would have been lurking and progressing over time.
Now in regards ***** ***** question and based on his signs, we would have to be concerned about neurological based issues localized to his brain. With his seizures, lack of awareness and collapse, this could be either a focal or diffuse brain issue. This means that we'd have to also consider of disease processes like bacterial brain infection, diffuse protozoal infection (E.cuniculi, Toxoplasma, etc), viral infections, head trauma (where a brain bleed or swelling was putting severe pressure on the brain), or brain based mass (tumor or parasitic cyst). And all of these could cause a normal rabbit to have a sudden increase in the pressure on his brain that could then lead to progressive unresponsiveness, slowing of respiration, seizures, and cardiac arrest.
Again I am very sorry to hear that Chase has passed so abruptly. These would have been our concerns for his signs with only some being treatable. And while his loss will be devastating, please know that he would have been feeling progressively sleepy and sedate and likely would not have been aware when he suffered those final seizures.
Please take care,