As I understand, bacteriophage are basically viruses that attack bacteria. They are strain specific, such that one type of phage will only infect a very specific type of bacteria. That's why you might have noticed some suppliers offer varying "types" of otherwise the same culture - like TA61 and TA62 for thermophilus culture. A cheesemaker could switch between the two strains to keep phage in check.
When bacteriophage infect a bacteria, like most viruses, they "reprogram" the bacteria to produce copies of themselves, and then basically make the bacteria explode, releasing more phage, to infect more bacteria, to produce more phage...and so on. Hence no bacteria to convert lactose to lactic acid and drop the pH. Hence a failure of the milk/cultures to acidify being a symptom of phage infection. But since acidification is happening, it seems the lactic bacteria are working more or less just fine.