The shark swallowed the spatula from the house after it ended up in a landfill. Unfortunately, the supernatural forces got "Amityville" mixed up with "Amity Island" and sent the shark towards the wrong location, where the Brodys happened to be. Life's a bitch that way.
Which sounds more plausible? A shark is starting to go around killing specific people out of "revenge" (due to a possible voodoo curse). Or that an old woman is starting to get an unhealthy fixation with these animals after losing a few of her loved ones? Her behavior throughout the movie shows that her mind has just about become mush at this point. The shark that ate Sean was completely different from the one encountered in the Bahamas. The fact that the shark in the Bahamas doesn't really seem all that fixated on the Brody family and seems happy enough to eat anyone it can get its jaws on doesn't lend any credibility to Ellen's "revenge" theory at the beginning.