I am so sorry about the loss of your birds. That must have been truly devastating! I don't believe that you did anything wrong. Birds can have many of the health problems that occur in any other species such as: kidney disease, parasites, liver disease, endocrine disorders, cancers, infectious disease, behavioral problems, nutritional deficiencies, etc.
They can be sick for a long time before you notice that anything is wrong. Hiding illness is a very important inborn behavior to hide from predators. Therefore, each of the birds that you lost may have been harboring illness for quite a while before they passed.
I had a bird 10 years ago and introduced another immediately that I brought home from the humane society --they lived happily together for several years until the complex I lived in decided to spray for bugs and didn't tell us - we kept the them by the window in the summer because they loved it -- they were dead the next morning.
One more thing...Bojangles gave NO indication that something was wrong. He was healthy when I got him -- the person I had gotten him from had him for over a year and he seemed healthy for the entire time I had him. Bucky was healthy when my friend gave her to me -- they had her for years with no complications. As I said, I noticed the other day that she was not herself - feathers all messed up, she seemed to stagger around the cage and would 'freak' from time to time - seemed to slam herself against the bars, sometimes getting her wings stuck between the bars -- she seemed too weak to hop around and resorted to 'beak climbing' and often misstepping when she tried to get on a perch. That is when I put her back in her old cage and the other bird got very upset -- he is not social but began to screech and squawk when I took her out. Now she is gone and he has gone back to the quiet, sad, anti-social bird I first bought from the pet store. I am absolutely devastated )o=
A related discussion, New