[corresation]

You realize that the Boston bombers gathered normal fireworks, which they then used in a pressure cookers, right?

Some hysterical Facebook user saw that she had posted a picture of a bunch of fireworks (in classic statistical failings, so many are claiming that no one could possibly call in for hers given that there are many other pictures of fireworks on the tubes. Those people have no understanding of odds), and in the shadow of Boston felt that this was a critical thing to tell the police about. The police pretty much have to follow up on all such things, so they go and talk to them, again in the shadow of the Boston bombings, asking about Boston bombing type things (because street-level law enforcement is often looking backwards, assuming the same crimes are the new norm).

Yes, a picture of fireworks absolutely explains why they would ask about a pressure cooker. Whether she had previously been looking up pressure cookers or backpacks (if that is actually true at all) almost certainly has nothing to do with this, but she knew where to go with it for maximum interest.