1. HN has an influx of new users who are somewhat interested in technology and technology businesses, but do not have enough domain expertise to engage on discussion of technical subjects, or subjects related to startups, such as design, customer support, finance, laws (as in interpretation of legal code, not politics), etc. For them, it's easier to engage in political debate. [EDIT] As a secondary theory: politics is a subject which interests a greater number of people than an specific technical subject or business practice.
2. HN's format concentrates debate and attention on articles that get popular just after being submitted: because more pondered or technical articles take more time to get popular, they never reach the front page.
3. With no major shift in the industry in the past year, and with mostly the same players (all of which were implicated in the NSA leaks, for instance), legal issues sparked from executive and judiciary actions are getting more attention, because they make for fresher, more sensational news, and reveal unanswered questions.
That's a really interesting point. As much as I've generally enjoyed the quality of front-pagers on HN over the years, I am now wondering about all the really good stuff that never made it there -- whether because the posters didn't optimize the timing properly, didn't game the headlines, or simply posted material that took awhile to digest and sink in.
Overall, there's probably a strong correlation between material that makes the front page and material that this community considers upworthy. But timing plays a huge role. I wish there was some way to counterbalance the effect a tiny bit. I can think of a few -- most of which would, unfortunately, be just as likely to harm as to help the reading experience.