They failed after the plaintiffs had them in their custoday for some time and moved them several times. Note also the alleged failure more: "That's when the handle tore and the bottom of the bag broke, the Sagers claim."
Somehow I doubt all of that happened at once. I can't think of a time when I've ever had a plastic bag fail catastrophically like this.
If this gets suit very far along I can see at least two Failure Analysis experts making a lot of money. I haven't been to Pittsburgh in a long time but I'm sure that I could qualify as an expert on plastic bags in a relatively short time.