请帮忙分析一句话
-Why was he punished by his teacher?
-It was because he behaved so badly.
回答中,it was后跟了个连词becasue,感觉有点奇怪：
be动词一般作为系动词用在主系表结构,或者作为助动词用在被动语态和进行时态等中.
这里是主系表结构吗?如果是的,系动词后不是跟名词或形容词吗,也可以跟连词吗?