I got some soy milk on Saturday and it expires May 2017 (if unopened I presume). I went to open the fridge just now and I had a cup of tea with it no problems. I went to drink some now from the bottle and it has gone sour and tastes identical to normal cow milk when it goes sour. It smells like it too and has thickened. I have never had this happen before and I have bought soy milk for months. I am not sure how this happened. There is nothing I think that could've caused it. Some vinegar was put near the bottle?? Could that do something? I always refrigerate it with no problems.
"Safe" does not mean 100% nothing will happen, it means that, even if all assumptions are met perfectly, there is a tiny chance that you will get bacterial growth. Even more likely is that not all assumptions were met. Maybe there was a small problem in the production process - storage procedures assume that food is produced under nearly sterile conditions, but maybe your milk was just over some threshold of contamination. Or possibly, your fridge is a bit over the recommended upper limit of 4 C.
However the spoilage organisms got into a growth phase, they soured the milk. Throw it out. If the fridge is under 4 C (measure in the place where you keep the milk - the door is frequently somewhat warmer), it is unlikely to happen again anyway.