What are some reasons bacteria are becoming more resistant to antibiotics?

Bacteria will evolve the molecular structures in various components to become resistant to antibiotics. For example, if an antibiotic works by competitive inhibition (ie - the antibiotic binds to the active site of the protein in it's cell membrane instead of another element the bacteria needs to survive), the bacteria may evolve an alternate molecular structure of that protein so the antibiotic won't fit in the binding site.