i had not noticed it, but two things occur to me. 1. it might be a testament to the hygienic behavior of your bees -or- 2. if the SBB of your hives is a bit clogged up, it might just be dead mites coming through as things are stirred up.
don't know when you last cleaned your screen, but i was amazed at how much stuff can accumulate on there. pollen, bee and other insect parts, etc.