In case you didn't know, Shigaraki suffers from Excoriation Disorder, which is a compulsive habit to scratch your skin. Now two ways to alleviate this habit is to get rid of the issues in your life that cause you the most stress OR covering your skin to prevent yourself from scratching it.
Tomura scratching his neck while stressed. ... Much like a child, Tomura seemed unable to cope with his own feelings, instead lashing out and abusing both others or himself, tearing at his own neck when he becomes frustrated.