ACTIVITY_ASSIGNMENT Question

I want to know DB Table ” innovator.ACTIVITY_ASSIGNMENT” what conditions will add the signers. For example : Workflow  A–>B the mean is A approve to B . System add B in the db table “ACTIVITY_ASSIGNMENT” When B reject to A system add B and A in the “innovator.ACTIVITY_ASSIGNMENT” .I don’t know why B reject to A will add B again?