The trouble broke out when rival fans started shouting at each other outside the stadium.

trouble 是名词，意思是"麻烦、问题"，指令人困扰或难以处理的情况。 broke out 是动词短语 break out 的过去式，break out 意思是"突然发生、爆发"，通常用于描述冲突、战争或麻烦等突然开始的情况。