If your cursor happens to be placed over a link while the page is loading, the page preview for it won't trigger. Presumably this is because it's relying on mouseover events, and there is none in that situation.
However, it's pretty trivial to figure out if any links are already hovered at the time the listener is added, with :
I've recorded a video of it happening: https://www.dropbox.com/s/9vx9bqzgcte0044/Hover%20on%20pageload%20issue.mov?dl=0
The window of opportunity for this happening has been made wider by T176211: Page Previews could load less JS on pageload.