Well it could have gotten red leg or what it sounds me to what happened was that it got injured in a fight over food with the other FBT. I am sure that you have seen them jump at one another when the other sees the other eating. Atleast mine FBT's always lunge at the other when they see eachother eating. Matter of fact I saw the larger one chump down on the smaller ones leg once when they were both feeding WHICH may be the case in what happen here. My smallere one of the two ALWAYS lunges at the bigger one if he happens to leap near and land to where the smaller one is sitting. I would guess from the swelling of one leg it got into a fight and it got severe damage to its leg and that was part if why it died.
Sorry to hear that happened and wish ya the best
-----
Aaron aka Doofy
Knowledge is not power; APPLIED knowledge is power
What is Victory worth without sacrifice