Bacteria that were once stopped in their tracks by antibiotics now grow and reproduce unscathed. Too much exposure to the drugs. True, there are other reasons bacteria are overexposed.
Doctors prescribe antibiotics inappropriately, as do vets.
And patients use them incorrectly. But the most glaring abuse occurs in agriculture. It is standard procedure these days to dispense low-level doses of antibiotics in animal feed to promote faster growth. Not enough when you consider the costs. Antibiotic-resistant bacteria that develop in farm animals have a direct route to humans through food.