I'm thinking diet - rabbits need unlimited hay and just an egg-cup full of timothy-hay based pellets daily. Pets at Home is not known as the best brand either. In addition to pellets and hay, rabbits need lots of fresh leafy vegetables twice daily. Straw is not recommended as it can cause eye injuries. If she was eating her pellets in the evening it was unlikely to be GI stasis, but it's still a possibility if she never ate hay overnight.
Maybe there was an extreme in weather, a draught through the shed ect.
There might have been a predator or loud noise outside that literally scared your rabbit to death, being prey animals this is very possible.
Were they vaccinated? Against RHD2 as well? If not, I would get your other rabbit vaccinated now to be safe.
Unfortunately, no one can tell you for sure what your killed your rabbit and even with veterinary attention before their passing they may still of died. We caught my rabbit Bitten's symptoms very quickly but even over several months of slowly deteriorating she was never diagnosed. Sometimes life is just cruel.
I have you can find comfort in knowing you gave your rabbit the best you could, that they had a companion to live with, a big space to play and although two years is a fairly short time, at least it was well lived. Wild rabbits are lucky to survive that long, due to illness, predators ect.
Now, you may not be ready to consider it yet but be aware your single rabbit will require a new rabbit companion after they have grieved for a couple of months. I suggest going to your local rabbit shelter, who will help you choose a friend for them and aid in bonding the two.