You have obviously never seen/experienced a 'seemingly normal' person have an unexpected autistic meltdown.
I'm not saying this girl was having one, (I wasn't there so can't even begin to say what it was/might have been), but sometimes someone with autism can have a meltdown for no obvious reason to the majority of us. The reason may be something completely different, but either way, the decent thing to do, (if someone is trying to deal with the situation), is ask if they need any help and if they say no then just walk away.