We sometimes take out a relative's German Shepherd for walks. She's generally been good with other dogs, playing well with larger dogs and ignoring smaller ones, and only getting a bit aggressive when the other dog becomes aggressive first.
We took her out on her first proper walk in an area with other dogs after lockdown, and she started behaving aggressively towards other dogs in three separate occasions. After the initial mutual butt-sniff, she starts snarling and snapping unprovoked. We've not seen this sort of behaviour before.
She's a three year old German Shepherd, not especially well trained, and is very territorial - often barking at people who walk by her house even if those people are well known to her. However this behaviour never occurred on walks - she's often been very well behaved previously.
So my question is, should we continue taking walks where she's exposed to other dogs? How can we reteach her that other dogs are fine to be around?