Alright, if you're still reading along, we're going to be talking about what could possibly be big enough to cause Dom (played by Vin Diesel) to turn his back on his flame, Letty (Michelle Rodriguez) and the rest of his crew. As in prior installments of the franchise, the notion of the group being "family" is spoken about often enough that it might as well be a drinking game (every time someone says the word family, take a drink).
After the race in Havana, Dom is walking along picking up a flower for Letty and comes across
Furiosa Cipher (played by Charlize Theron). After about a minute of dialogue, she hands him a phone and tells him that he's about to turn his back on everything and everyone he knows and loves – to come work for her.
He watches a short video (that we don't get to see), and from his furrowed brows, we're to take it that whatever she just showed him was enough to tip the scales. A reunion with
Maui Luke Hobbs (Dwane Johnson) and a few scenes later and the crew have stolen an EMP bomb. Everyone splits to go their separate ways to meet back up at the safehouse, but Dom follows Luke and runs him off the road. Luke is trapped in the driver's seat while Dom leisurely steals the bomb and escapes to rendezvous with Cipher's flying headquarters.
While onboard we discover what she's got over him – his former lover, Elena Nives (played by Elsa Pataky) has been kidnapped and is being held on board the plane by Cipher. And just to make things more screwy, she's also got Dom and Elena's infant son. So, in short, he does what she asks, or she'll kill mother and daughter. Now, let's pause and remember that to the best of our knowledge, Dom and Elena haven't been together since Fast 6 when Letty showed back up. The baby is maybe 6-8 months old. So that means that for this storyline to hold up, no more than 15-18 months has passed since the events in Fast 6. Eh, that's a bit of a stretch given the time frame (unless we want to go with the idea that Dom's still been hooking up w/ Elena more recently).
So that's the big hook that forces Dom to abandon everyone. Now, granted during a scene where Dom and Cipher break into Nowhere (Mr. Nobody's headquarters), Cipher plants a huge kiss on Dom and he doesn't really resist. Well, it is Theron, after all.
And for the final emotional hook of the film, the baby's name isn't revealed until the very end – and it's Brian. In the fiction of the franchise, Brian O'Conner is still alive and just "retired". It was the character played by Paul Walker, who died in 2013 from injuries sustained in a car accident. So the naming of the baby as Brian is a nice and touching way to honor Walker's legacy.