<human>: Can you summarize the given text in a few sentences?

[Rhu] born April 2013, is a wonderfully obedient and loyal Border Collie Aussy cross. She is easy to train and is very well behaved, except one thing, starting at about 10 months old.

We'd meet another dog while walking. She would wag her tail playfully and walk to meet the dog, seeming friendly. She would approach the dog face to face, still friendly. This was when it would go wrong. Often she would freeze for a couple seconds, nose to nose, staring the dog down. Then she'd go off at the dog without provocation, as if a switch had gone off. We'd pull her away snapping and snarling, which often provoked more aggression. By the time we were about 10 feet away she'd forget about it, walking along as if nothing had happened.

This, of course, confounded us. As experienced dog owners we'd never seen this before. The most puzzling part about it was both the suddenness of it and how she was so friendly to start. We could be throwing a ball in the park with other dogs not 30 feet away and she'd be fine with them (we don't do this anymore, just in case she changes her mind).

We bought a halty and a muzzle and are now very careful with her. We took her to a trainer, who conditioned her for dominance, showing us how to be alpha. No dice. We took her to another trainer who though it was fear, and tried to re-socialize her with some of his dog. Again no dice. At this point we really don't know what to do, it is manageable as is, but it would put us at ease if we could train her out of it. I realize there is probably very little I can glean from reddit that I wouldn't get from 2 professional trainers, but I though I'd give it a try.

To be clear; Rhu gets lots of exercise, including bike rides, runs, walks, hockey and balls (though the off leash activities have been somewhat limited since this new behavior arose). She never chews or riots in the house and loves people.
<bot>: Dog (1 yr Aussy Border Collie) is super friendly with people, very obedient and trainable. Fine with other dogs until she gets face to face, then goes off. 2 trainers, no dice.