I wouldn't be surprised if it had something to do with paper storage. My darkroom is located in the basement near the boiler and it can reach 85+ during the summer. I normally bring all paper back upstairs at the end of a print session but there have been times I've left it down there for a day or two.
I'm still confused why I don't see this with the 11"x14", which is a much older box and stored in the same way. The 100 sheets of 8"x10" were purchased much more recently, I couldn't say when i first noticed the specks but its been months since I first remember spotting it.
Maybe a bad batch of paper. As mentioned above it's not visible in any way once the paper has dried.