When the trending topic is associated with an event, or something which is covered widely on the news, then clicking on the trending hashtag in Twitter does show some news source for it. But, if the trending topic is obscure then it will be a hard time finding why that hashtag is trending as clicking on the trend would just lead to tweets which includes that tag. Of-course, this is just w.r.t Twitter I presume finding why something is trending on other social networks except reddit are not straightforward.

As far as twitter is concerned, their data consumption API is flexible for reading tweets and anyone can implement this as a feature in a 3rd party website/app; but there is no such API for Facebook and so this need gap needs to be addressed by Facebook itself.