I have had the Squirrels knock over several of my Ferns. I had to put make shift cages around some of my really small ferns. We used the green Bamboo Steaks with regular twine to cover the tops of them.

I also had a stray cat that wandered into my yard last summer.(I guess he is a pet now) He loves laying right next to my Ferns. He never lays directly on them just up against them. I have noticed that since he showed up the center of my larger Ferns seem to fall over. I think the little bit of pressure he does put on the ferns is enough to bring the centers down. It's a shame he is so friendly it would make it easy to chase him off for destroying my ferns if he was a mean guy!! So it could be a neighbor cat or stray cat that is knocking them down.

Maybe try to put a natural barrier, like old sticks from a tree or something similar around the base to keep it safe and see if that helps. If the Ferns are big enough you might be able to conceal the barrier enough so it doesn't look tacky. Hopefully it would be enough to keep animal away from the base.

Good Luck!