LONG POST. How deus ex machina applies to Jurassic Park's final raptor/Rex moment:
Short answer: it's doesn't. The term gets wrongly applied to any old bit of contrivance, but in real terms, a deus ex is used so rarely in modern storytelling that it practically doesn't exist. Audiences wouldn't stand for it.
On a literal level, the T-Rex escapes on-screen in act 2, then clearly chases after prey - humans, then other dinosaurs. At a key point it's shown having left its island area entirely and gone after a flock of gallimimus. So we know it's roaming the island, hunting. It arrives at the end and does what we knew it could do, in ways we knew it could do it. (We'll ignore the question of how it quietly slipped into the building.)
A thing happening that's predicated on prior set-up isn't a deus ex machina. Ever. That's the opposite, in fact. If it's established as possible, as happening already, the term is not applicable.
If you want to argue that the moment is dissatisfying, I wouldn't blame you. It's rarely a great way to end a narrative, good fortune.
It bothers me less, I think because if it had been a human character, we'd have bought it. There's no shortage of "We freed this guy in act two, and he fled...then showed up in act three to save us" in movies.
And part of that is this: the Rex is a victim, and a hero, not just a threat. Genre hands you some conventions that are free to use without crossing the line into deus ex. He's never attacked by our characters, only run from - he's an antagonist, but not a villain. In movie terms, we want to see him. We look forward to his arrival. The audience will him back. The kids befriending the herbivores, and Alan and Ellie respecting the power of what's been done here, counts. There's a meta-text at play.
But that meta-text, unlike an audience willing a god to intervene and emerge to save the hero, is based on established textual information - that the Rex is loose and hunts - not simply hope and faith.
On a grander level, Jurassic Park's gods ARE people. That very specific "man destroys god" conversation makes it clear: these animals are our creations, and they serve us and threaten us. Like any creation. Hammond put the Rex there, Nedry set it free, the humans led it out of it's bounds. Man-made incident, man-made animal.
Heady nonsense, really. But genre and theme hand you certain narrative devices. In farce we allow a broad run of coincidence, in horror we accept unrealistic lighting choices, in an action movie we allow for an above-average tolerance for injury.
In a monster movie, we let the monster show up where it likes - soundlessly if that's more fun.
Still, if Jurassic Park's final moments were based solely on genre and theme, one might still call deus ex machina. But thankfully the Rex was clearly established and justified.
His arrival is a coincidence for sure. But that and deus ex machina are not interchangeable terms. It's not "a coincidence at the end". Critics like to throw the term around that way to justify dislike - "It's not just that I found it a bit unlikely, it's that the writer literally broke a fundamental rule. Their professionalism is at fault." But really, one shouldn't break out the Latin so easily.
Which of these would be a deus ex at that final raptor-attack moment?:
1. The computer hacking included some things being activated that Lex didn't intend - including handily sending down a lift at a key moment.
2. Ian Malcolm bursts into the room in a jeep and knocks the raptors flying.
3. Muldoon, bloodied but alive after all, strides in and blows them away with a shotgun.
Answer: none of them. They're all entirely within the bounds of the film grammar, genre and behaviour we've seen to that point.
What none of those would be, I'd argue, is good writing.
The arrival of the Rex isn't good writing either. It's unlikely, clunky. It ends the threat on a beat of coincidence - though it's important to note that the threat remains, in the form of the Rex, so fleeing still remains the priority.
It's not good writing. But it's not deus ex machina. It's not a previously undramatised, previously unliteral god arriving out of the machine of narrative to save the unsaveable.
And it IS good cinema.