Their marriage started to get a bit rocky due to the mother's devotion to finding Mary.

rocky 是形容词，由名词 rock（岩石）加上后缀 -y 构成，字面意思是"多岩石的"。在这里用作比喻意义，表示"不稳定的、摇摇欲坠的"，形容婚姻关系出现了问题。