Light Spots on Clothes After Washing?

For years, once in awhile a load of laundry will come out with random lightened spots on them. I do not use bleach so I know this isn't the problem. And I'm even using Woolite now hoping to solve the problem. I also allow the detergent to mix with a good amount of water before putting the clothes in. It seems to happen with dark tan and brown items, sheets, towels, and some clothing. What is causing this? And how can I prevent it?

By K8 from Los Angeles, CA