Sometimes, each group is Good. People who are Right about a subject, will agree and want the same things to happen in that field. For example, the US, Israel, and Australia all want the same thing to happen in Iraq.
Another way it can happen, is for Logic Of Situation reasons. For example, "anti"-racist groups and secular humanist groups, both find the logic of their situation, as anti-American groups during a discussion of War on Iraq, to imply they should make asses of themselves opposing the war.
Of course, there is the conspiracy explanation, but this is generally a very bad one. This claims that they are secretly not independent groups. A real world example might be various terrorist organisations and various terrorist harbouring states. But you only call them independent, and think their links don't exist, if you are really silly.
There is the luck "explanation" which is true on rare occassions.