The number one cause is the "left hook". This is when someone is turning left and doesn't notice pedestrians in the crosswalk moving across the street the driver is turning onto. Unless there is a dedicated left-arrow green, these pedestrians almost always have the right of way, but the driver doesn't notice them because they are too focused on the oncoming traffic or other hazards, and completely forget to check. I've attempted a crude ASCII diagram. C is a car with a green light, attempting to turn left, P is a pedestrian walking up

          | | |
          | | |
       ____   ____
       --P-   ----
       ‾‾‾‾   ‾‾‾‾
          | |C|
          | | |
          | | |