。
All the trees had shed their leaves.
Literal
Trees [topic-は] all leaves [object-を] dropping had-finished.
落としていた is the past resultative of 落とす ('to drop, to make fall'), the transitive partner of 落ちる ('to fall'). Trees here are doing the dropping. Past resultative ~ていた tells us the leaf-dropping had already happened by the reference time — they were already bare. すべて ('all') is a more written-style equivalent of 全部.