Dog suddenly aggressive toward other dogs on leash

My golden retriever has always been friendly with other dogs but recently started lunging and barking at dogs we pass on walks. Hes fine off leash at the dog park but on leash its like a switch flips.

Hes 4 years old and this started about a month ago. Nothing traumatic happened that I know of.

Has anyone dealt with leash reactivity that developed later? What helped?