Here's why:

* Decreasing distance: This indicates the two objects are getting closer to each other.

* Constant or slowly changing bearing: If the bearing (the angle between the two objects) remains constant or changes only slightly, it means the objects are moving on a path that will eventually intersect.

Examples:

* Head-on collision: Both objects are moving towards each other with a constant bearing.

* Crossing paths: Two objects are moving at an angle, but their bearing remains relatively constant.

Important Note: A changing bearing can sometimes indicate a collision will be avoided if the change is significant enough. However, if the bearing changes slowly or remains constant while the distance decreases, it's a strong indication of a potential collision.