"Since bacteria are extremely numerous, random mutation of bacterial DNA generates a wide variety of genetic changes. Through mutation and selection, bacteria can develop defense mechanisms against antibiotics."

https://health.mo.gov/safety/antibioticresistance/generalinf...

Mutation happens all the time because cell replication isn't perfect, some mutation are irrelevant, some deadly, some bring better chance of survival.

It's not a response just the result. Or how does the bacteria know it's an antibiotic and not just water? It doesn't, water just isn't a evolutionary filter, antibiotics are.