Karma is action that is generated by desire arising from the source of the individual ego, the aspect of mind that thinks of itself as separate from creation. The mind uses the senses to reach out and touch the creation, and in coming into contact with it, tries to force it into generating the fruit of its desire, what we want in our lives.
This behavior is the source of suffering. Desire-based activity is the absolute cause of transmigration from one lifetime to another. The karma doesn’t play out fast enough for us to keep up with it in a given incarnation. It tends to accumulate. We only fulfill a very small percentage of our desires within a lifetime.
So the karma builds up inside your system and you quite literally get caught in this time loop where the entire thrust of a given lifetime is to redeem karmic coupons that you generated maybe five to eight centuries ago when you were incarnate in the Dark Ages and your life was really full of ignorance and superstition.