"Do you know if that was natural corrosion that caused those to stay in, or simply friction, or do I need to use some sort of adhesive when I reattach them?"


Just friction. You'll need a press to get them back in, which is just some allthread, big washers, and a couple nuts. The really old school way would be to tap them back in with a hammer and a block of wood, but that is too likely to result in a wrecked headset or frame.