The fence also turns out to be shoddily built, so this isn't necessarily a stunning example of the superhuman proficiency of the sheep at fence-busting. It's mainly important because, for some reason, there have been tons of skeptics who didn't believe sheep would ever try to escape from a fenced pasture because the sheep would understand the shepherd had intended them to stay inside it; that they'd be bred for obedience, because they're sheep; that strong fences weren't necessary because sheep are dumb; that liability for damages to neighboring farms would in any case be sufficient to ensure shepherds build sufficiently strong fences; that even if sheep escaped, they couldn't do anything without opposable thumbs; etc.
Whereas this outcome was obvious for years to anyone who believed that sheep graze on grass, and if you don't put the grass inside the fence they will keep looking harder and harder for an exit until they find one, and the exit they find might not be where you intended the gate to be.
So now the latter group are pointing to it and saying "see? See? I told you the sheep would try to escape!", because to them it's a clear early example of what they've been worried about, while the former group have moved on to saying "well, obviously they should have built a stronger fence; anyway the sheep barely caused any damage once they got out", "the sheep didn't break the fence, they're just animals, we should really see this as the shepherd using sheep to break down their own fence", and so on.
This is a true story, but may also be a fable.