Problem towing a trailer

[BelgianPup]

I was following a pickup with a camper shell today, and it was towing a smallish utility trailer. The trailer was weaving back and forth.

I have never seen this problem on the road myself, only in online videos.

Two questions: 1) What causes this? 2) What can you do to prevent it?

 

[independent]

For how it happens I can only say what I've seen from the videos and demonstrations with models (of the toy variety).

Like this

Weight distribution and overall weight is my understanding.

 

[Carla618]

Similar video:

 

[Gr8ful]

I didn't watch the vids but would guess the trailer was tounge light, too much weight in the rear.