The answer is the antibiotics could unintentionally kill good bacteria living in your intestines, leaving you more susceptible to infection by pathogenic bacteria.
Antibiotics will indiscriminately kill any of the bacteria they come into contact with that have similar characteristics to the pathogen being targeted. This can cause trouble because many helpful bacteria live in your large intestine. Antibiotics target bacteria and not viruses. If you take antibiotics it is important to restore any good bacteria in your gut by taking probiotics at the same.