Garden Fence Slat Movement?

[DrWho]

Had a wooden fence erected spaced slats using neighbours posts, a week later I notice one of the slats that was straight before but all of a sudden is squint and leans more towards the next slat, in other words there's no space at the bottom when there was previously, none of the others are like this.

And how I know for sure that the slat wasn't like this previously is that I luckily took a picture which took in the part of the fence I refer to.

The neighbours fence slats have no spaces erected solid, could this have anything to do with this?

 

[footprints]

Wood warps particularly fence panels (which are not the best quality wood as a rule) just put up some featheredge myself and some have gone wonky, luck of the draw really.