when electric wire fails

Why does electric wire fencing stop working? When vines overtake it. When grass grows up in it. 

We use electric fence on our perimeter and to form temporary paddocks for the cattle and the pigs. The fence line at the very back of the property is overgrown by weeds...we swear it wasn't last week...and shorted out the fence charge. The livestock got out. Repairs done, but it will keep growing, that's how things work...