This can be due to a few different reasons:
Poor Placement: if it was installed without an attention to detail and the vapor barrier was installed without the edges overlapping as they should and water will rise over the edges
Critters Chewed Threw: the crawl space environment is inviting to many critters, and if they need to, they will chew through a simple 6 mil vapor barrier to get into this space. Once chewed through, water can seep up through the hole
High Waters Occurred: If water levels get high, the water can move the 6 Mil vapor barrier out of its place. Once the vapor barriers have shifted, it can be easy for water to creep onto the barrier.
Condensation/ High Humidity Issues: moisture content in the air can allow the water from the foundation to vaporize and condensate, dripping the water on top of the vapor barrier.
To stop this issue, a full interior drainage system with an accompanying sump pump can make a big difference so water levels cannot rise above the vapor barrier. To fully protect the crawl space though, a full encapsulation will deter critters, is sealed down into place, includes a dehumidifier, and will set your home up for success for the long run.