Out of context, it looks like Starlight is staring at the audience and giving the “coo coo” gesture at RD’s randomness.

I distinctly recall seeing an edit of this scene where she actually brings Spitfire back with her…

If a sonic rainboom is a rare phenomenon “when a rainbow and a sonic boom happen at the same time,” you gotta figure they happen separately more often than not.

@c0c0c0c0  
Which fanfiction? Headcannons change depending on the narrative, so you can’t really say anything for sure except for what we have been shown. And as it is we were lead to believe that breaking the sound barrier is synonymous with a rainboom.
 
Edit: Just saw your /s Ugh.

@c0c0c0c0  
It’s still a fanfiction.

@Eeveeinheat  
You have obviously not read the fanfiction, EDUCATE YOURSELF FOOL! /s
 
Pegasi use magic to fly, kind of like unicorns, the magic levels they have tend to differ from one another, RD is part of the 1% of pegasi who have a stupendous amount of magic (kind of like Twilight), these pagasi are called speedsters. Now how does this correlate with their flight speed? Speedsters can go at mach one and more easily, normal pegasi have to struggle to get to mach 1-2, which is usually the maximum they can achieve.  
So what what makes RD so special that she can do sonic rainbooms, while other speedsters can’t?  
Sonic rainbooms require speeds that exceed mach 10, but normal speedsters usually cap at mach 5-8, RD can do more. Rainbow Dash is a very special pony.
 
Just a small comparison. The average bullet travels at 2,500 feet per second (around 1,700 mph/2700 kph), mach 1 speed is 1225 kph, mach 2 is 2x mach 1 and so on. RD is faster than a bullet, way faster.
 
PEGASI MASTER RACE!

@Toejam  
Obviously, but in Equestria a sonic rainboom is their equivalent to a sonic boom. Unless they were misleading us this whole time?

SOUND barrier, not rainbow barrier!

But if she broke the sound barrier, where was the rainbow?