Shaving can cause irritation to all parts of your face, but your neck is a particularly delicate area - and the most likely to show signs of redness and irritation.
Your neck is a surface of curves and angles making it hard to shave smoothly.
Neck hair tends to grow in a lot of different directions, so it’s tricky to catch all the hairs in a single stroke of the razor. You have to make several passes – with more chance of irritation at each additional stroke.
Couple that with the fact that the skin on your neck is very sensitive, and you can see why this area can be a problem to shave.