Simple: I Spat Fire.
"Hey Spitfire, how exactly did you get your nickname?"
Spitfire sighed, and looked at Rainbow Crash. "I thought I told you that you didn't want to know? It's crazy, and, to be honest, a little embarrassing."
It was a couple of days after the tour: Rainbow Crash was still cleaning everything up around the area, and had run into Spitfire - luckily not literally - and decided to ask, despite being told that she 'didn't want to know'. It had only made her more curious.
"Oh come on! I know everyone else's story behind their nicknames, and they're all pretty funny! And you know mine, so I want to know yours!... U-Uh, Ma'am." Rainbow stuttered a bit near the end - Spitfire could kick her out of the Wonderbolts at any time; speaking that way to her was probably a bad idea.
Spitfire just rolled her eyes a bit. "If you want to know how I got it, that's simple: I spat fire."
Crash blinked. "You... spat fire? Like a dragon breathes it?! Pardon me for asking, Ma'am, but... how?!"
Spitfire did another long sigh, and remembered everything like it was yesterday. "It's a long story, Crash."
Finally! Spitfire had finally gotten into the Wonderbolts!
She could barely contain her excitement: all those years, those long, hard years of training to be in the Wonderbolts had finally paid off... and now? Now, she was going to fly with them for the first time! Finally!
Todays the day, Spitfire thought. She could barely contain her excitement; it was a bit unlike her, but she just couldn't take it! I'd better make a good first impression: show them how good I really am!
Spitfire took a deep breath, and gave her wings a stretch. It was time to show the Wonderbolts that she belonged.
She pushed herself into the air and flapped her wings hard - a great way to get her moving fast as soon as she took off. She flew up high into the air.
It seemed that she had already caught the attention of a few of the Wonderbolts below as she flew higher. Finally, she zoomed downward, gaining more speed as she quickly did two loops in the air, as fast as she could to impress them.
Then, Spitfire threw herself upwards again, going straight through a cloud, and punching a perfect hole through it as she flew up more. She did a spiraling motion, and even shouted 'Woo-hoo!' into the wind in her face.
Today was the perfect day.
She smiled and looked down - just about every single Wonderbolt there was watching her now. She felt like the happiest pony in the world...
And then, it all came crashing down - literally.
Suddenly, Spitfire started racing downwards - she panicked. She hadn't meant to do that. She had stopped flapping her wings when she looked down, and the way she angled her head angled her entire body to look down. Because of this, she was crashing down hard.
She gasped and struggled - now was not the time to screw up! Every single Wonderbolt was watching her! It was time to look cool, not look like a fool!
In an attempt to fix everything, Spitfire adjusted her angle; because of the way she panicked, she was very unsteady. However, she did correct herself, and she was now flying perfectly in line with the cloud, which was just few inches below her. She turned and gave a nervous smile and a wave at the Wonderbolts that now seemed to be staring at her instead of watching her.
Because she wasn't looking where she was going, she crashed straight through the cloud-hill in front of her, and into a rainbow waterfall.
Spitfire came out of the waterfall covered in rainbows, and did a bit of a spin as she crashed into the clouds. She groaned.
She looked up to find all the Wonderbolts crowded around her and gasped.
Unfortunately for Spitfire, she had crashed through the Rainbowfall with her mouth open.
Spitfire's face turned red as a tomato, and orange as a flame. She found herself sweating as the world around her seemed to heat up, until she felt like she was melting in a furnace. The very important thing that she had forgotten about rainbows, was that they were very-
"SPICY!" Spitfire cried out, and ran around in circles, desperately trying to spit out the hot, spicy, multicolored liquid in her mouth. She felt a bit like a dragon breathing fire.
Finally, after about five minutes of panic, Spitfire calmed down. She looked up at the Wonderbolts...
...And they all burst out laughing.
"Spitfire!" the Captain of the Wonderbolts called out, desperately trying not to laugh, "WHAT was that reckless stunt you just pulled!?"
"Her name is SPITFIRE?!" One of the Wonderbolts called out, "Oh, that's just TOO perfect. It's Spitting-Fire, everyone!"
Everyone laughed even harder as the now nicknamed Spitting-Fire blushed.
"Please?" Crash begged for Spitfire to tell her the story. It seemed that Spitfire really didn't want to tell it.
"Sorry, Crash. I still wish I hadn't pulled such a reckless stunt, even if it did earn me my nickname. So, if I don't tell ponies about it, it only exists in my memories."
Rainbow Crash just begged one more time. "Please, at least tell me how you 'spat fire'?"
Spitfire sighed. Finally, she said, "...Did you know that rainbows are incredibly spicy? You'd never guess until you tasted one..."
Rainbow laughed. "Oh, yes I know! My friend Pinkie Pie got a little tour of the Weather Factory and tried to taste a rainbow when she saw them. She turned every color of the rainbow after that, it was pretty funny to watch. Are you saying that you spat fire because you drank a rainbow?!"
Spitfire smiled. "More then that: I splashed through it, drank it, and basically inhaled it too."
Rainbow laughed a little more. "That sounds crazy!"
"It really was. Anyways, I have a long day tomorrow, I've gotta get to bed. 'Night, Rainbow Crash."
Rainbow winked. "'Night, Spitting-Fire."
Spitting-Fire laughed a little, and headed back to the baracks for the night.