Perhaps they built the log trap on a known AT-ST patrol path? And then led it down the path on purpose (after all, the walkers WERE chasing ewoks through the woods)?

The only argument about it being far fetched that I think really holds any water is the need to get the timing of the cut right so they swing together accurately.

But even that wouldn't be terribly hard to do. If you hung them at the same height with the same length ropes then you'd just need to cut them at the same time. The timing wouldn't even need to be exactly perfect, just close. And how easy is it for humans to synchronize actions? Pretty easy (one, two, three, GO!!). I'd hardly imagine ewoks couldn't do the same.