Really? to me hn leans extremely conservative. There was even a twitter account called shithnsays that had, many a quote from the community.
I think it comes both from a love of rhetoric and from a point of view that being a contrarian can make you appear smarter, or more interesting.
I know I've been guilty of such behavior.
I did not follow the Roe vs Wade thread specifically, so my view may be out of touch, but from what you describe it seems to be closer to this kind of behavior rather than actual conservatism