What the heck?!?

Have just socialised my dog Rosie with another dog and they looked like best friends, playing and such. But last night and again this afternoon Rosie tried attacking this other dog. Had to take her home the second time.
Any idea what changed? The first attack she ran up to him and tried attacking him, this time she was tied up and he came up to her and she lunged at him. I don't get it

Probably needs more than a 1 time meeting to socialize with another dog. There could be allsorts of reasons for it. Jealousy, territory maybe scared especially when tied up and cant defend herself. You need to let her meet the other dog a few times on a leash and praise her when she behaves. Maybe a treat for rosie when she behaves and a treat for the other dog too may help

Nah they've met many times before both on leash and off. This was out of the blue. She has been showing a lot of dominance over him by mounting him (not humping) and putting her paw on his back. Mind you this is at the male dogs house.

I'm sorry I have no advice although I'm sure many others will be on soon with experience in this area

Are you sure she was trying to attack? They play very rough and can also be quite vocal. Suki plays with an Akita and it sounds and looks like they are fighting but they are great friends. Here is a link that you may find usefull on body language.

http://staffy-bull-terrier.co.uk/dogbodylanguage.html

Well the other dog yelped at the top of his lungs and ran so I don't think it was playful

What exactly happened? I can understand the one where she was on lead & the other dog approached - on lead dogs can feel quite threatened by unsolicited approaches.

What were both dogs doing when she turned?

The first time I wasn't watching. A friend was. The second time I had her tied up a few metres away cuz they had friends over and didn't want her jumping all over their baby and 3 year old. On the second occasion she was tied up and the other dog came up to her to say hello, his tail was wagging happily. They were smelling eachothers noses and next minute Rosie lunged at him

Obviously, neither of us can really say what happened the first time, but the second time I think she may have felt 'cornered' as it were. She was tied up, there were strange people around & a bit of commotion.

I wouldn't worry too much, just keep a bit of an extra eye on her the next time they meet.