My kale plants keep falling off of my hugelkultur patch?

I started my first hugelkultur this year, and planted kale and silverbeet plants on the top of it. At first they seemed good, but now every few days or so I find one or several of the plants laying on the ground beside the patch. I keep planting them back on the mound, but at this point I think most of the plants aren't going to make it. I'm very confused. My only thought is that it could be birds (very large crows here), because I also am having cabbage worms on the plants. Could the crows be pulling the plants up trying to get the worms? I also have tomato plants on one end of the mound, and they seem to be fine.

Or could it be due to erosion on the mound? Wind? Any thoughts? I'm pretty sure no one is messing with me and pulling them up, nobody goes back there.

How thick is the soil on top of the wood? If there isn't enough soil, the roots may not have enough purchase.

How tall are the plants when they fall over? Some of the more vigorous brassicas can get pretty top-heavy and need staking to keep from tipping over, especially if the soil is rich. On a steep slope like a hugelkultur bed, that tipping might be enough to make them fall down.

You could try staking individual plants, or putting in some heavy duty stakes at either end of the bed and running twine or wire to support them, like a trellis.

Try stabilizing the mound soil a little lower down by replanting them about midway up instead of on top.  On a fresh mound there can be quite a bit of sloughing until things get stabilized especially on a really steep mound. Oh and mulch the heck out of that sucker!

Another thought....the cabbage moths could indicate a stressed plant also, and unless you are providing lots of water to the top of the mound that area tends to be pretty droughtie even in wet weather.  That peak just dries out really fast, I like to plant stuff like sage up on the top and reserve the prime growing areas for stuff that likes a little bit more stable moisture.

Are there any tracks around your hugel bed?  I've seen cattle pull out plants (pine seedlings) and thow them into the air.  You're probably fenced in, but do you have any wild critters who might be playing?  Australian equivelant of deer, maybe?

It has to be critters. You might try covering them, even if only at night. I do this frequently when I have an unexplained problem like this that I don't have time to suss out. I use yogurt containers, cardboard boxes, or the most convenient: a bit of old row cover (agribon or remay) or even an old thin sheet, cut into appropriate sized pieces you can make little domes out of . Very handy also for season pushing.