Why did Dr. Manette again make shoes and then quickly return to his normal self?

Dr. Manette was wrongly accused and imprisoned in the bastille for many, many years. In order to keep himself sane he began to cobble shoes. For some reason, this simple act allowed him to remain 'normal' and to remember who he truly is. He returns to cobbling shoes when he cannot emotionally accept the things going in around him..... and for this reason, he quickly returns to his normal self.