答案:Both goats want to be the first to cross the bridge and they begin to argue loudly and angrily.
问题翻译:他们为什么在桥上打架?
根据文章第二段第11-13行:
But both goats want to be the first to cross the bridge. They begin to argue loudly and angrily.
但两只山羊都想第一个过桥。他们开始大声而愤怒地争论。
可知,答案可总结为:Both goats want to be the first to cross the bridge and they begin to argue loudly and angrily.