The idea of people from one country going to another to serve in their military is, as others have pointed out, not new. The reasons people have done have also varied, from simply seeking money or fame, to ideology to religion.
The reasons states are willing to employ mercenaries also vary, but usually relate to a lack of willing/available native recruits given current military needs.
For example, the Byzantine Empire experienced a dramatic shortage of soldiers in the late 11th century following defeats in the Battles of Manzikert and Dyrrhachium in 1071 and 1081, and a civil war in the intervening years. The new Byzantine Emperor sent out a call to various West European monarchs and the Pope, seeking mercenaries to allow him to fight back against the Turks. And he got quite a few. But his call also led (by a somewhat complex process) to Pope Urban to call for an armed pilgrimage to Jerusalem, which we know better today as the First Crusade.
Others have mentioned the Spanish Civil War, and the International Brigades were a fairly extensive source of manpower for the Republican government, at least early in the war, although the proportion fell as more native Spanish were conscripted. In this case ideology was a strong factor, but it also had its drawbacks - the Communists hated both the anarchists and the anti-Stalinist Marxists (the POUM which Orwell joined) and red-on-red violence was a serious issue.
I've read only a little about the white mercenaries who fought in various conflicts in Africa (mostly) in the 1960s, but I get the impression a lot of the attraction was thrill-seeking by ex-professional soldiers. I also wonder if they had a bit of a sense of superiority over the poorly trained local soldiers they fought with and against, and perhaps felt their job wasn't particularly dangerous.
Australia also has a share of people who've left the country to fight in Syria. The first of these date to some time before ISIS became a news item. However in the last few months pretty much every report of people leaving Australia involves them going to join ISIS.
One thing which has struck me about the stories is that quite a few of the people involved seem to have had ties to organised crime and drug rings. I don't have the details but I get the impression that recruiters have been seeking out Muslim youths who've gone badly off the rails and offered them a path to redemption. In my opinion things aren't helped by a segment of the general community, egged on by irresponsible sections of the media and some ignorant comments from politicians, who have a negative attitude to all things Islamic, thus giving a free rein to new ISIS recruits to say they feel disaffected and not wanted in or part of mainstream Australia.
The other thing which probably hasn't helped is having police raids involving hundreds of officers which results in the arrest of a couple of people and virtually no charges being laid. It's easy to see that people wanting to inflame tensions can present these 'overkill' raids as the Muslim community being targeted by police and politicians.