I too belive that someone did soemthing to her outside & it probably involved a loud device. Just one more reason I never leave my dogs outside unattended.
There are a few different ways to help w/ this fear but without knowing the exact cause it may be difficult. I would talk to a behavorist &/or reputable dog trainer.

Whatever you do do NOT praise or re-assure her or talk in baby voices when she is scared, that will just re-inforce the bad behavior.

In the future I would make sure she only goes outside supervised that way you can assure that nothing happens again. Especially now that is starting to show progress.

Maybe leave yummy treats & toys around the yard to incourge her to get out of the garage.