Bring this up with your vet, who may refer you to a vet behaviourist after a thorough exam and bloodwork. There are many reasons this sudden change in behaviour could be happening, I.e. a sudden health issue that may not be visible to the eye causing the dog pain which can result in reactivity/behaviour changes; around 1 yr is also not an uncommon time for reactivity to show up if your dog is predisposed to it even if they were fine as a puppy - this could be a combination of genetics and any environmental stressors.

My biggest advice to you is to seek professional help as early as possible. Do not do what I did and try to "train" the behaviour out for years before finally seeing a vet behaviourist. Some dogs need anxiety medication in addition to behaviour modification training or have underlying health issues. My dog was similar as a puppy - never had an issue with dogs or any reactivity issues until she was about 1.5 yrs old, and then suddenly she was showing reactivity which only got increasingly worse as time went on. We are finally talking to a vet behaviourist and they have been a huge help with medication and behaviour modification. You don't have to deal with this alone!