我们这样来分析一下这两个句子：
它们可以分别用一个非限定性从句来改写而不影响原句的意思——
Their car was caught in a traffic jam,which thus caused the delay.
The song is sung all over the country,wihich makes it the most popular song.
由此可见,它们中间的非限定性从句都是用来对主句的补充说明（即用来修饰整个主句）,而前面的主句便成了从句的主语.
如果这样还不易理解,再看原句还可以改写成下面的形式而不改变意思：
Their car was caught in a traffic jam,a fact that thus caused the delay.
The song is sung all over the country,a fact that makes it the most popular song.
很显然,a fact 就是用来指代前面的主句的,同时也是从句的主语.
你句子中分词结构的逻辑主语分别就是整个主句.
顺便说一下,凡是上面一类的非限定性从句都可以改写成 -ing 结构而不影响原句的意思.