In fact rebirth is caused by the result of pending karma itself.
Jivatma will get body only when he has good or bad karmas in his account :) These good and bad Karmas don't get cancelled by one by another. One has to suffer/enjoy their both karmas. When the balance is zero that Jeevi will not born again; thats nothing but Moskha/Muthi. Which can only be achieved by offering the karmas to the God.
This is the reason why Lord Sri Krishna insisted in abandoning the result of actions:
BhagavadGita 2-51
Karmajam buddhiyuktaa hi phalam tyaktwaa maneeshinah;
Janmabandha vinirmuktaah padam gacchantyanaamayam.
-> Endowed with spiritual intelligence wise men giving up the results arising from actions certainly liberate themselves from the bondage of birth and death attaining the state of complete tranquility
The theory of Karma is not so simple to understand.
I would like to quote from one of Sri Samavedam Shanmukha Sharma's discourses on this topic:
"The results of Karma (may be good or bad) will not be given all of it at once to any one just like a mother who cooks the exact amount of food needed for her child but not the entire bags of rice that they have stored in the house."
