There are several very obvious reasons why the Iraq war has jostled to a position at the center of the 2016 conversation. One is that the rise of the Islamic State in the region has made foreign policy and the handling of Iraq an important point of debate for possible candidates. Another is that one of the Republicans who is likely to run is the brother of the president that pushed for the invasion in the first place. Jeb Bush's unpopular answers to questions about the war have given his opponents plenty of opportunities to attack, reinforcing that Iraq would be a popular topic of conversation.
But the subject was likely to come up for a third reason: 2016 has a relatively large number of likely or possible candidates who were in Congress when the authorization for the use of military force (AUMF) in Iraq was approved in 2002.
Overall, fewer than a third of the members of Congress who voted on the AUMF are still in Congress. Of that group, slightly more voted for the resolution -- that is, for the invasion -- which generally aligns with the overall result of the vote. (We used GovTrack.us data from the House and Senate votes for these calculations.) Two current members of Congress who also are or might run for president are included in that 161: Sen. Lindsey Graham (R-S.C.) and Sen. Bernie Sanders (I-Vt.). Graham voted for the war resolution and Sanders voted against. (Both were in the House at the time.)
But a number of former senators are also maybe/definitely running: Hillary Clinton (yes), Lincoln Chafee (no), Rick Santorum (yes) and Joe Biden (yes). It's the most presidential candidates who participated in the vote since the large class of 2008. (Of course, there was no contested Democratic race in 2012.)
A number of the other candidates were in the happy position of not having had to weigh in. Searching news archives for Marco Rubio, Ted Cruz and Mike Huckabee doesn't reveal any stated position, though as Republicans, it's likely that each supported the authorization. (Chafee, we'll note, was one of the few Republicans -- he's now a Democrat -- to vote no.) Rubio and Cruz have said this week that they wouldn't have invaded Iraq given what we know now.
As long as Jeb Bush is in the race and as long as the region is unsettled -- the latter of which, at least, seems like a safe bet through 2016 -- the Iraq war will be part of the presidential conversation. And for a number of candidates, that means explaining their old positions. Or: their brother's.