その木はひとりでに倒れた。

that (something or someone distant from the speaker, close to the listener; actions of the listener, or ideas expressed or understood by the listener); the; um...; er...; uh...

tree; shrub; bush; wood; timber

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

by itself; automatically; naturally

to fall; to collapse; to drop; to fall senseless; to be forced to bed (by illness, etc.); to die; to go bankrupt; to be ruined; to have a bad debt; to be defeated (in a game); to fall (of governments, dictators, etc.)