It’s likely nucleation. Basically, the water wants to boil, however it can quite get over the hump to produce a gas bubble. When you add salt, the surface acts as a nucleation point. Once the bubble forms, it grows rapidly, often splitting and forming more.

You see this effect at the other end too. Supercooled water will remain liquid, until something becomes available to crystallise around. When this happens, the whole lot will freeze in seconds.