After the accident, he began distancing himself from his family as a coping mechanism.

distancing 是动词 distance 的现在分词形式。distance 本意是"距离"，作为动词时表示"使保持距离"或"疏远"。 这里 distancing himself 表示"使自己与他人保持距离"的行为，即主动疏远家人。