Easy. The zombies are caused by a fungal infestation (similar to a real fungus that causes zombie-like behaviour in ants), after time spent wandering, the zombies would set root, as their bodies are transformed into giant mushrooms.
Ok, it looks shit written down, but I swear, it could create one of the most sinister zombie subtypes ever! Imagine it, if you will, a forest of decaying flesh, trees that look like they might once have been human, slowly moaning, their flesh punctured by bizarre growths, the sun blotted out by the spores of this malignant plague...
It's the best bits of zombie apocalypse and body horror combined!