It's not a straight conversion to aggression. Germans, I believe, drive fast. Maybe aggressively too. But, if you put them in an auto, they also pick up the behaviour of building speed fast for 1-2 seconds then braking hard. It happens a lot in dense, halting traffic. It's not the only way of being aggressive, but it's another way

People don't do that in a manual because it's not fun.

*In Italy people drive mostly manuals. They also drive like suicidal grandmas in fast forward.