They keep on messing with the ingredients in these so that animal rennet (taken from the stomachs of slaughtered baby calves) is often used. Walkers crisps in the UK kept doing this.
You also need to avoid their meaty flavours you need to avoid as some of them like the smokey bacon and roast chicken contain REAL meat. Yuck!
They may not contain animal by products, but they are filtered through isinglass, which is taken from the swim bladders of fish. There is no way to ensure none of this enters the product.
There are veggie versions like Free From.
This is because it contains Parmesan (see above). Pesto is often put in so called vegetarian dishes by restaurants. See this article.