dog was aggresive then friendly

Question
You answered one question for me recently and I have another. Your awesome by the way.   Tonight my friend brought her 20lb. fox terrier over to visit my aussie shepard/border collie mix. (Sierra is friendly, but is not fond of small children) She is my rescue ,age two, have had for 6 months, gets along well with my big male collie moose.
Anyway, "Jasper" came into the back yard and Sierra saw him through the window.  She began to growl and bark, and not in a friendly way like she usually does.  I decided to leash her and take her out instead of just letting her out.
She seemed very aggresive towards him. (He was not)  I was getting scared but said, lets leave the yard and go for a walk on neutral territory.  Should I have done this from the beginning?  We walked and she continued to be aggresive but not as bad as in the yard.  We came back in the yard slowly, each time letting  them sniff each other.  Jasper lifted his leg, she sniffed him, and then she started wagging her tail.  We let him off the leash first and she was wagging her tail, no longer growling or barking.  When I left her off the leash, they were instant friends.  And played and played for two hours without a single problem.
Why do you think Sierra acted so aggresively when I have never seen her act that way in six months.  She came from a home with many dogs and never had a problem.  Never had a problem with my collie either.  Was it the surprise element of seeing another animal in "Her " yard?  Maybe because this dog was so small?   Thank you so much.  Debbie

Answer
Hi Deborah,

Dogs take in information with more than just sight. She saw a dog in her back yard that she did not recognize and could not smell or hear well.

I think I would have leashed her too and all turned out OK. But a leash limits as well and if attached to her collar her head could easily been pulled up into a fighting posture.

Better to meet neutrally, go for a walk and then go in the backyard. That way they have time to take in the sight, sound and smell of each other without the social pressure of territory or actual contact.