Given the task definition and input, reply with output. Given scene/plot descriptions from movies or TV shows, and given a specific question about the scene, summarize the question into a title

Although there is no disputing the impact that Steven Spielberg's Jurassic Park had when it was released in 1992, there has always been one scene that has bugged/perplexed me.  I speak of the climax, when Alan Grant et al are surrounded by the Velociraptors and about to be pounced upon, when suddenly the T-Rex lunges in and 'saves the day'.  It has already been established earlier on the film that the T-Rex telegraphs its approach with thunderous footsteps, and I'm pretty sure that even if the humans were surrounded by malevolent reptiles, they would notice a huge T-rex squeezing in through the front door of the reception center.  Possible explanations include the T-Rex's ambushing skills (as evident in the earlier Gallimimus chasing scene), the afore-mentioned distraction of the Velociraptors, or even a huge suspension of disbelief to better serve the dynamism of the scene.   I don't like the idea of T-Rex just being shoehorned into the scene to add some va va voom, so is there a better explanation?

Where did the T-Rex come from?