I’m very sorry for your loss.
Rabbits are very good at hiding signs of illness. Prey species do that so as not to attract the attention of predators. So it really is very hard to tell sth is very wrong, the signs are very subtle. Eating less is a warning-sign though. It is like Lunavenus says about stress and bunny tummies: Stress (like the thing with the cat) can cause the GI-system to slow down, the bunny eats less and that in turn makes the GI system slow even more. Undigested food can build up in the intestines and cause a partial blocking, the gut slows down even further. Cause of death in your buns sounds like it might have been GI stasis, ie a full stop of all GI movement. It’s unfortunately a very common cause of death in bunnies. Him not being able to drink his last night could’ve sped up the process, but this was sth that most likely had been going on for quite a while.
If action is taken early on when you see poor appetite and reduced amount and size of poop, it’s often possible to save the bunny (but unfortunately not always). But you need to know what to look for and how serious it is for a bun. This is not really common knowledge. You don’t really see this in other pets like cats and dogs (although cats can get blockages if they get dry feed and don’t drink enough). We try to raise awareness here, and by posting about your bun, you’ve helped doing that.
Again, I’m very sorry.
Binky free, little bun.