what are the social condition in spain that makes this possible...
and what social group do the people come from
I don't think it's so easy to make such an easy link between social condition and ideology here. It's known that some of these ultras belong to "good families" without huge economic problems. Of course, you may find anything and it's possible that some of them come from the forgotten parts of the society.
In any case, I think it's more related with people who need to belong to a certain community, where they feel strong in the mass, to fill their individual weaknesses. Once I read a book about a jounralist who entered in Ultrassur to know them, a dangerous task, if you ask me. The idea he defended is that, when you're in Ultrassur, you feel strong, unbeatable, protected, powerful. The ultra-right elites, who never shave their heads, use them to achieve their political goals. At the end, the ultra groups aren't a goal themselves, they're just a tool to spread their propaganda in an attractive scenario like it's football. Many kids get attracted by the chants, the flags and all that... and they finally "capture" those who need to satisfy their social needs. This may be more evident in the kids that live in problematic quarters, but it's known that others already are ideologized before. At the end, there was a long ultra-right dictatorship in Spain until not so long ago and, even though this idea is minoritary (they're not even in the Spanish parliament, unlike other countries of Europe like France, Italy or Austria), they actually exist. They're children of old politicians, policemen, member of the old army...
Said all this, I think it's the clubs' responsability to accept them in the stadiums. Some presidents get the protection (via chants, banners, etc.) of the ultras in a shameful way and some others still have this naive idea of the ultras as kids that don't know what they do. In a way or another, I think that ignoring the reality of the ultras is a huge mistake.
I don't think it's a casuality to see that the biggest problems appeared in stadiums that have large ultra groups. It happened everywhere, including the Camp Nou. If you ask me, Laporta's greatest job was to kick them out of the Camp Nou to make them irrelevant. Florentino tried it too, but I think that he lacked a last impulse. In his recognition, the Ultrassur are less than half the group they were (see them in the 80s and early 90s and you'll see what I mean). On the other hand, look at the Biris in Sevilla, the Betis Supporters, the Frente Atletico, the Osasuna ultras... whenever these groups are bigger, the bigger the problems are. It's an evidence. You'll see how nothing happens in Villarreal. Never.
It's true that there are some ultras that link with the ultra left, as it happens with Osasuna or Deportivo. But this is a different reality. They normally link with radical nationalist ideas too (like the ultra right groups), although hidden in ultra left ideas.
For me, ultra-left and ultra-right are very near when it comes to