Can pasteurized milk turn into yogurt by itself?

During the winter, I often leave milk on my porch to make room in my refrigerator.

During a recent warm spell, I had an unopened gallon of pasteurized milk on the porch for two days around 50 degrees Fahrenheit. When I opened the bottle to see if it had survived, it had no spoiled milk sour smell, but the consistency was similar to thin yogurt. I drank a little, and it tasted like normal unspoiled milk aside from the texture.

I was to nervous to consume the whole gallon, so I threw it away, but I'm curious about what process might have been at work here. Could it have been a yogurt like bacteria despite the pasteurization?