I restarted, but the icons were all still visible (about half are usually hidden right after startup). I managed to resolve the issue by going into taskbar properties, customize, setting one icon to "always" hide, closing out, then opening the properties again and changing back to my original settings ("hide when inactive").
But now the problem is back, and it took several tries of the above fix to get back the normal behavior.
Does anyone know why the system tray would stop hiding the systray icons of its own accord?
Thanks.