That is a scary pony! Her facial expression doesn't say 'rage' per se; more like 'You're gonna die now."
I always imagined it might be something she learns to control in the future and this would be from around that time (she'd probably be protecting somepony). I'm hoping they'll bring it back up in the show at some point.
I'm pretty sure the show version is nothing more than a sight gag, but I like the idea that it's a power she'll actually gain control over in time. In other words, here's hoping that I'm wrong and you're right!
"What was that, Mr. Trump?"
I can't help but see this as . Her face when fire puns?