I figured out what triggered it...
...I'm an idiot.
I rarely give the dogs rawhides/bully sticks/etc they usually get stuffed Kongs and reusable treat toys. But...we got rawhides for the 20 dogs that were here over the holidays, so they'd be more chill. So today I gave Inara, Sawyer and Rip a rawhide while I took the other dogs out to work with me. When I came home, I put the leftovers (they barely chewed on them) up on the counter. Inara was guarding them...and I was about to go put them safely away.
I just remembered though that Vesta's former foster mom said that rawhides are the ONE thing that she'll guard.
When she had one at Christmas, she was safely put away in her crate (as were all the other dogs).
Now I know how serious she is around them...they won't be given out anymore while she's here! At least I figured it out and it's a clear trigger.
"I don't have any idea if my dogs respect me or not, but they're greedy and I have their stuff." -- Patty Ruzzo
"Dogs don't want to control people. They want to control their own lives." --John Bradshaw