What can be done to avoid bacteria mutating to resist antibiotics?
The real answer is nothing. Bacterial resistance is a natural phenomenon that current science has no way of preventing. The other answers that others have listed are only ways to slow down the process.
Why did the antibiotics cause it to mutate?
Bacteria, like every organism, wants to survive so it allows for mutations to occur in its DNA. These changes may make pathways of certain antibiotics ineffective against the new form of bacteria