Why is it a common trend to pan the camera past an object then back to it?

This has to do with communicating to the viewer what the intended subject is, when the subject may be ambiguous among other things.

A hunter is searching for his prey. A red bird sitting in an apple tree. He pans past several apples, then the bird and then returns to the bird. It's now clear that the red bird is not a red apple.

While some viewers may identify immediately that the bird is not like the others. There are still some who missed the difference. Panning back allows that item to be reevaluated by the viewer a second time.

It's based upon the idea that people sometimes have difficulty identifying slight differences in subjects. Sesame Street made a popular game for it called One of these things is not like the other.

While the technique has become over used. It is just a simple way for the film to highlight a subject. It's similar to techniques such as lighting an object separately from the rest of the scene, or suddenly changing the music when an object appears.

It's a mark or queue to the viewer that something is important. In this case, it's done with the camera movement.

In addition to Mathew's answer, it might also be to emphasize that the searching person is indeed, well, searching and doesn't initially know where to look exactly. Directly moving its view to the target might on the other hand seem like he knew where to look all the time. So introducing this slight delay of recognition emphasizes the searching process.

And in addition to that this particular effect is indeed usually used in cartoons, which are in the end famous for exaggerating and over-generalizing things.

In addition to Matthew's and Christian's answers, I would add that this is a specific form of the old "double take" trope; in this case the second take is being shown through the observer's eyes, as opposed to it being shown from an external POV.