how does poor time management or inadequate planning lead to plagiarism?

Answer 1

Just as you asked. Poor time mgmt and inadequate planning usually means you will not get your project done on time and to the best of your ability. The stress of wanting a good grade or at least to turn in something will temp one to put down someone elses work thus plagiarism.

Answer 2

If you don't manage your time well, you're likely to rush through what you are doing! When you're in a hurry, it's hard to come up with well-thought out, original ideas, so you're more likely to just re-write what someone else says. With adequate planning, you can read/view another's information, think about it, and re-state it in your own way, possibly adding some of your own original thoughts to the information!
Hope that's what you are looking for in the way of an answer!