Can anyone please tell me why?

I would start with the bearing. I only takes a little wiggle at the center to throw the wheel of by more than an inch from top to bottum. I can tell just looking at the pic that the top of the tire is definitly leaning inwards towards the trailer. A quick check on the bearing is take it for a quick drive. Then feel the hub with your hands. If its a bearing going out that hub will be alot hotter than the wheel behind it. If they are about the same temp then I would guess axle. I say this from experience. I was double pulling my camper and boat last year and had one bearing blow on the camper on the way to the lake and one blow on the boat trailer on the way back from the lake. Was a very expensive weekend.