Why? Many dogs feel very trapped when made to sit when they are worried about an approaching dog, so they just lose their marbles. Think about it from the dog's perspective - most sane people see a dog blowing up, growling and freaking out and they move away, which gives the dog exactly what they want - to be left alone, not having to confront another dog or person. A perceived threat is approaching, the dog blows up and the threat goes away - mission accomplished. It is just too much pressure for some dogs to be expected to sit calmly and quietly, when on the inside and in their brain, they think what's approaching is going to harm them or their owner.
How to Make it Better? Walk slowly and methodically with your dog, give gentle leash pops as needed interruptions and encourage your dog to look to you for guidance - you have the keys to life management and your dog must begin to realize that you will intervene on his behalf and keep him safe. After all, we ALL want to feel safe. Talk to fellow walkers, many of them probably have walk challenges too, set up practice sessions frequently until your dog is more comfortable and confident on the walk and builds trust and faith in you.