I am not a smart or experienced at all, but this is my guess.

someone turning right into that top street and slowing down

Maybe because they are driving past parking spaces, and concerned someone might back out?

someone coming out of that street also slowing down

Based on the image where the truck is, there is another road very close to the top one. If the people driving there aren’t paying attention or try to rush the roundabout, the top people would have to slow down.