Hi Mary and welcome
My guess would be that since Swiss Miss is made by Con Agra--a company that will disclose any gluten on the label--they were warning of the chance of cross contamination.
Quite possibly, the tapioca was processed in a facility or on the same equipment as gluten.
I think it's great that they put this on the packaging, so people can then make their own decision as to whether or not to consume it.