Best guess would be an antioxidant. There are a number of antioxidants often present in polypropylene. Generally, it is assumed that they won't extract out. This is not always the case. One common antioxidant I ran across is BHT (butylated hydroxytoluene). There are many others.
There are many other ingredients in polypropylene. These can be polymerization catalysts, inhibitors, lubricants, etc. Any of these is also possible, although the antioxidant is more likely.
The problem will be getting the plastic manufacturer to tell you about the materials they use. It's proprietary information that is rarely disclosed.