Seriously dudes.. I’m surprised none of you have got what caused it..!
It’s caused by bimetallic corrosion of the inside of the steerer due to intimate contact with the star nut.
The star nut scrapes the anodising away when it’s bashed in, and gets a fair amount of spray up from the wheel in winter, with lots of yummy salt. The steerer has eroded away from the inside. It’s the same with those pace forks. Look at how rusty that crown race seat is.
It’s best to take the top cap off occasionally and squirt some spray grease round there, or better still, never use a star nut in the first place on an aluminium steerer. Hope head doctors rule.
Go now and take your top cap off and have a look… You’ll be appalled at the state inside!