Why is my dog growling/barking all the sudden?

[Rusty Shackleford]

Maya turned 2 years in March. Last summer, she was cool as a cucumber when we all sat on the porch.   All of the sudden this summer, she’s occasionally growling at passersby, cars pulling in/out of driveways, and people down the street walking AWAY from her. Yesterday I had in in the backyard and she started barking (I honestly thought it was another dog) and she ran towards the front because the mail lady was there.  She knows her so when she got to her she immediately sat and waited for Michelle to give her a treat, lol (gotta love mail carriers who carry treats!). But the barking and running was brand new and very concerning. 

She doesn’t lunge at people (other than the above incident) and when someone’s coming to the door, or if we’re on a hike, her tail’s wagging while she’s growling and she moves towards them submissively. 

The interesting part is she’s only doing it when we’re outside with her. If she’s out alone, she’ll watch the UPS guy walk up to the door with no incident. So it seems like it’s a protective thing. I’m playing email tag with our trainer, but figured I’d ask you all what your thoughts are on the reasons and any tips you may have. 

Maya is a certified therapy dog who comes to work with me everyday. She doesn’t do any of these behaviors at school, but I want to nip this ASAP. 

Thanks!

[J and Riley (and Luna)]

Weird lol.

I mean I guess it probably doesn't matter what you use as long as you're consistent.