Any reason twisters (tornadoes) spin mainly counter-clockwise in the Northern Hemisphere?

Answer 1

This is because of the Coriolis effect.

Answer 2

low pressure systems spin counter-clockwise due to balance between the Coriolis effect and the pressure gradient. Pressure gradient force acts towards the low-pressure area and the Coriolis acceleration acts away from the center of the low pressure. Instead of flowing down the gradient, the air tends to flow perpendicular to the air-pressure gradient and forms a cyclonic flow.