To answer your first question, Identitarian is a right wing political belief from Europe:
The Identitarian movement is a pan-European socio-political movement that started in France in 2002 as a far-right youth movement deriving from the French Nouvelle Droite Génération Identitaire.
Here's how the splash page of /r/Identitarian defines itself:
Identitarianism is a movement which approaches the idea of an indigenous european identity from a reactionary and rational perspective. It recognises the importance of european culture, history and values. It seeks to promote a pan-European ethnic consciousness and to protect the quintessential character of European society. PM for access, this environment is too risky to let anyone participate
As far as why the term is trending, well the movement has been in the news a few times recently:
http://www.telegraph.co.uk/news/2017/04/08/angry-under-30s-driving-french-far-right/
http://www.dw.com/en/german-right-wing-identitarians-becoming-radicalized/a-38032122
http://www.sbs.com.au/news/dateline/story/dateline-europe-special-young-hip-and-far-right
Also in Breitbart:
http://www.breitbart.com/london/2017/05/04/young-first-time-voters-explain-choosing-le-pen/
With the upcoming election between Le Pen, a favorite of some of the Alt Right, and Macron in France, more people are talking about French politics.
Additionally, with the fall of /r/PussyPass as an alt right subreddit, many of those folks are seeking a new home on Reddit. As a result, you'll see discussion about this more often than usual in subs like the_donald. Here's more info about that drama:
https://www.reddit.com/r/PussyPass/comments/684bow/nazis_lose_again_never_won_anything/
https://www.reddit.com/r/pussypassdenied/comments/685xs6/the_nazis_of_rpussypass_have_been_removed/
Therefore activity is spiking in those related subs, as well as their subverse on voat.