They might drop leaves out of time, or not blossom when they should, or wither without cause--and when even trees cannot live, this probably expresses that the conditions for human survival are also pretty atrocious.
若不該落葉卻落葉，該開花卻不開花，無緣無故枯黃，連樹也不能活時，可能代表人生存的條件也很惡劣了。