QA 36 reactivw 9 MO. GSD

Hi

I have a 9MO Female GSD that gets triggered whenever there are 2 dogs nearby playing or whenever we take a walk with one of my other dogs, she gets triggered instantly without warning whenever she sees another dog.

Just some extra details.
No bite history.
Social 1-1 in general, whenever she gets evasive I don't force her to interact or shoo the other dog away, but never had problems with 1-1 interaction.
No problem's with me, extremely social with other people in any situation i've been in.
She's partially trained, my biggest mistake with her was I didn't engage with her as much during her younger years.
No problems in house, she plays a bit rough with the older one but he corrects her.

I don't need her to be super social in a group setting, but I do need her to not go nuts at every walk we do together with my other dog or whenever she sees 2 or more dogs playing.

What would you think the problem is and how to solve it?

Thank you for the reply

I'll definitely take a look at leash reactivity but she's fine and while still has some issue with engaging, she engages and rarely has problems with during our walks (if she's alone). I'd reckon after doing some research its a bit of redirection aggression, I have bought Michael's video on aggressive an dominant dogs and in the process of watching it through and will be purchasing leash reactivity when I can.
Unless you have any other suggestions for videos other than these 2?

Thank you in advance.

Okay, will definitely start watching it tonight. Thank you Cindy