In short, it's evolution. Bacteria develop resistance when exposed to antibiotics. They lose their effectiveness.
This will give you a decent overview of antibiotic resistance:
http://www.who.int/mediacentre/factsheets/antibiotic-resista...
If you like to watch documentaries, Frontline has a few on the topic, though each show is several years old now which is an eon to pathogens
https://www.pbs.org/wgbh/frontline/film/hunting-the-nightmar...
(on youtube https://www.youtube.com/watch?v=gE4VzFz9PPo )
https://www.pbs.org/wgbh/frontline/film/trouble-with-antibio...