Why must you brake now?

You have to brake because the green vehicle creates an immediate conflict with your path at the junction. Even if you are on the priority road and normally have the right of way, you must not insist on it when another road user is about to enter your lane and a collision is likely.

Rule of thumb for these situations in Germany: right of way is not a “right to drive through.” At intersections you must always drive in a way that you can react to typical mistakes by others, especially when your view is obstructed by other vehicles. A vehicle waiting at a side road can pull out late, misjudge your speed, or be hidden behind another vehicle. As soon as you recognize that someone may not yield and could cross your path, braking is required to prevent an accident.

The motorbike and the yellow car are not the reason according to the official solution for this question: the key hazard you must react to is the green vehicle.