My budgie Cicero suddenly died out of nowhere

I have a budgie named Sparky who has been alone since I’ve gotten him. I wanted to get him a friend this past summer, so I went to the breeder I got Sparky from and got him a friend named Knox. Before I even got a chance to introduce Knox to Sparky or take him to the vet, Knox broke his neck while I was at school and passed. I decided to try again, as Sparky seemed pretty lonely. I went to a different breeder and got Cicero. He was not at all tame and always bit me when I tried to tame him, even with millet. About 11 days after I got Cicero, I left for school on September 29, 2023, and he was sitting on his highest perch and seemed fine. I had to stay late due to a football game, as I am in the marching band, and when I got home, I sat down for a few minutes then decided to put my animals to bed (at the time I also had 7 mice, a snake, a leopard gecko, isopods, 3 birds including Cicero, 3 dogs, a cat, and a freshwater snail). I turned Sparky’s light off (he can’t be covered like my caique due to night frights so I never covered the other two budgies so they were used to it), and looked into Cicero’s cage to see how he was doing. I didn’t see him at first so I figured he was at the bottom of the cage playing like Knox used to do and Sparky sometimes does (not because of illness). But when I opened the cage door, I found him dead at the bottom without any warning. I checked for a broken neck again, but all the bones were in place, and there was no evidence of any other injuries or sickness. I also cleaned the cage very thoroughly after Knox passed before placing Cicero in the cage and always cleaned the paper, perches, and food and water dishes. By the way, Cicero was only 3 months old. Because of this, I have not been able to get Sparky a friend because any other budgies I get keep dying, and it’s very frustrating and sad. Does anyone have any idea why he might’ve died?