Yeah, they could be related.

Her response could be an anxiety / fear response.

Does she have heightened startle responses at other times? When cars backfire, or when there is a sudden noise more generally?

It could be that she has a heightened anxiety / fear response that is really kicked off in response to loud noises.

Typically... People habituate. The body can't keep up an intense stress response for too long so eventually it gets tired and the fear lessens off.

But some people have responses that are very intense, very readily triggered / set off, and are very slow to habituate / return to baseline.

Does it seem like that is what is going on?