Several have mentioned possibilities. Bushings and bushing dust is what I suspect.
Another or two:
Are you staring with a square blank? If so do it sometimes catch a little, maybe for a split second - once or twice - before you get it rounded? These friction catches are and fine metal powder from that friction can work it way to the end on lighter colored woods.
Are you taking the turned blanks off of the mandrel to inspect them before finishing them? If so, then this is where the migration from the ends to the outer edges of the end takes place. Hands holding the end will move the dust to the outer edges like it was grease.
Are you using any kind of liquid - DNA or accelerator? DNA will bring the dust up if you are not careful. It may not be noticeable at first but it happens. Accelerator, when there is not enough CA on the ends, will do the same as water or DNA.
I use turning between centers and occasionally get brass dust on the ends. I am careful to clean the ends BEFORE finishing. Usually, I will give a light sanding to the ends which will smooth the ends even more and also remove any sanding dust layer that builds up on the end.