The word karma means action, the action that produces effects or consequences. We’ve come to think of it as what is produced, not what produces. What we call karma is traditionally called “the fruits of karma”, i.e. the fruits of action.
Lingustic confusion aside, the Buddha said that the cause of karma is its own karma. The consequences of our actions comes from our choices, and these choices are based on attitude and ways in which we think. If we remain the same we will produce the same sort of karma over and over again, and this will produce the same sort of results.
Karma is not what comes back to us, it is what we produce. And what we produce (individually and collectively) we have to live with. If you want to change your karma, change the way you live your life and the way you think.