Some plastics do have chemical reactions with each other over time. It may be that the plastic/rubber feet have reacted with the plastic tupperware container.

I had some Inkadinkado acrylic stamps that I took off the original acetate packaging and put into a CD case for storage. The two reacted with each other, and the stamps melted into a nasty silicone goo which got all over everything.

I'd love to know what response you get from Sizzix.