Hi
If by wear on one side you mean the flank of the saddle, the vertical part, then that is abnormal. I can't suggest the cause, although I doubt it's just one load of rain. Saddles aren't that flimsy.
If you mean a hollow on one side of the sitting area, that is probably you. Those bones poke into the saddle hard and for a long time, soany difference in their prominence will make that side dent and the other not. That is the benefit of a leather saddle: it changes to your shape.
I can't say why you should have noticed this dent so soon after the rain. It may just be that you were looking for damage and spotted something that was there all the time.
There's a lot of difference between a bone-accommodating depression, which a lot of people have, including me, and a saddle that sags. It will only sag badly if it's allowed to. All leather saddles have an adjustment beneath the nose, either a big clumsy bolt on a Brooks or a dinky allen key on a Berthoud, and they're there for a reason: the reason being to tighten the leather as it stretches.
It stretches most when the saddle is new. The stretching will then slow down or even stop. But let it go too far and the hollow will be incurable and the sides of the saddle will flare out.
Remember: the saddle people put the adjustment there for you to use it.
happy days
léo
