At 2 years old he's also entering full maturity. My Jibini never started fights (he wouldn't even react with aggression if he was threatened by an aggressive dog) until he was 2-3 years old. Before that, I took him to the dog park all the time & used to marvel at how he would react almost playfully to aggressive behavior from other dogs. If another dog threatened him, he'd run off almost as if he was getting a kick out of it.
Sure enough- it was fall, the year he turned 2 years old, the first time he ever postured & got snarky with another dog. By the time he was 3; he wouldn't always start fights but he'd also never back down from one. At that point our dog park visits were scaled back- we went only during off-peak hours OR when I knew it was a "good" dog park & there would be a group of other Basenji owners present. Basenjis do tend to play better with their own kind HOWEVER snarks & fights do happen. Most of the time it's all bravado and no bloodshed- it sounds worse than it is. Occasionally you'll find a couple of Basenjis who simply will not get along.
Jibini has also historically had an issue with "big black dogs" too…I have heard the same from other B owners and I don't know what it is about big black dogs they don't like?
Most notable was his reaction to a big black Standard Poodle belonging to a very nice lady I was hoping would be able to take care of Jibini for a few days. It was the most extreme, vicious, nasty reaction I've EVER seen out of Jibini towards another dog. It started the moment Jibini laid eyes on the dog from 20 feet away- he hit the end of the leash, teeth bared, mouth open, horrid guttural snarls- VERY unusual for him. Usually he just postures, snarks, has a bit of rough-and-tumble and it's over. Rarely does aggression continue once he's established himself. But for some reason, Jibini just HATED this dog....went for him every chance he got, with everything he had...and this poor sweet Poodle just cowered and showed submission and did everything he could to appease Jibini. It didn't work- Jibini just kept going after him with more fury than I've ever seen. Even after walking them around the neighborhood together & trying everything we could to foster harmony, I had to find someone else to take care of Jibini for me at the last minute....and Jibini got along with the other person's dogs (all Basenjis) just fine.
Leashed dogs are more likely to be on the "defensive". Not only are they in your presence & may exhibit "resource guarding" behavior, but the restraint from the leash makes the dog feel less able to flee to defend himself....therefore, he's more likely to be defensive if he feels threatened. The methods outlined in Click to Calm, Control Unleashed & other books are often quite helpful in reducing/eliminating on-leash aggression.
If I were you I'd work with him on-leash using positive reinforcement for focus & attention- and otherwise restrict his access to other dogs, especially off-leash when you can't control him (or the other person's dog). Good luck