In real life, fireworks are reserved for special occasions. But authors add them to books all the time. In this particular instance, fireworks is a euphemism for the writing that makes a lot of authors uncomfortable: sex scenes.
You don't have to focus on romance writing or erotic novels for these to spring up. I once got a lot of feedback that I ought to add some sexual fireworks to one of my books -- a tragic, tear-filled historical. So they are definitely going to creep in no matter what the heck you're writing.
You're a Firework
It's a common saying that sex sells. If Fifty Shades taught us anything, it's that people like to read about sex. So if you've got some hot-and-heavy love interest in one of your books -- be it a screech-inducing horror novel or a sweet tale of love -- your characters might begin to drift toward this seemingly inevitable conclusion.
That puts you in a pickle. It's embarrassing to write about sex. It's embarrassing to talk about sex. I've gotten embarrased thinking about sex. And writing takes a certain brand of fearlessness if it's going to work, so for many authors sex scenes turn into a bit of a problem.
The key is that you've got to find your comfort level. There are tons of very clever ways to allude to sex without actually putting readers inside the bedroom. There's a way to write about everything that will allow you to stay comfortable; you just have to find it. It may take a lot of re-writing and re-thinking, but there's always a way for you to create what you want in the way that you want. Use metaphors and declarative writing, rather than descriptve writing, to put your point across.
Fireworks can be a small, short burst, or a soothing fizzle. They don't always have to scorch the page. Write about it in a way you can feel good about, and don't be afraid to add sex to your books. For every author, there's a different "right way" to do it.