Dumb luck or --
Mind you, putting your weight on dead wood in the forest is always foolish, especially your foot, since you don't know how sound it is.
But it's really hard for something to happen just by accident in the fantasy novel. Meaninglessness is not a good thing in a plot, because it's really, really, really hard to work with.
On the other hand, that would impose some requirements on the malice they do face. Maybe there is a malignant force in the forest, maybe some of their enemies cast it. . . . Perhaps it was accidental side effect from an earlier spell.