I think PakmanUSA made an excellent point - there's always individuals that do these sorts of things when their country is at war. A couple of Irish for example in World War II was given, but there were actually anglo-english people that were vocal about their support for Hitler. It's not really anything signficant, and to be expected that a small minority are going to do some stupid things.
Regarding the ones who are brainwashed. They do exist. It's difficult to say whether they're Pakistani or Indian in origin, for example, there's many Indian Muslims in Tablighi.
However, I think things that push them in this direction are
1) Poverty, feeling of alienation.
2) Import of illiterate Mullahs and Maulanas.
3) Failing themselves, looking for some sort of sanctuary.
4) Perhaps more, can't be bothered to fill this in yet.