Temporal distortion is the result of anachronisms, tat is temporally inappropriate items left in the past. There are 2 types of anachronisms that can cause such disturbances:

A simple anachronism are very minor and temporary alterations to the past, and are often inavoidable even by the most careful of agents. These cause no noticeable effect on history.
Effective anachronisms involve the result of more significant changes to the past, more enduring than the first. There are two kinds and both are taken very seriously, likely caused by deliberate historical sabotage.
Limited effective anachronisms are the changes whose effects to history are not great. Their repercussions (temporal distortion ripples) fade with time and do not alter the course of history.
Expansive effective anachronisms are changes to historically significant items or events with major repercussions or chain reactions (reality distortion waves) that make sweeping changes to history as they grow with time. When they reach the present they make unmake persons or give them new lives, completely unaware of the original timeline.