I had this on my egg that didn't hatch, my baby was breech and had died in the shell. I hope someone can give you an answer to this, as I'd be interested to know what it means too. Good luck with yours.
I don't think the other chicks generally damage the other eggs. It kind of encourages them to hatch, being tapped and peeped at, maybe it just happens after a certain amount of incubation, I think the shells tend to get more brittle later on anyway as the chicks inside have absorbed calcium out of them. Give them another day or two, they may just be late hatchers.
Thanks I think I will do that. This is just day 21 now but I got worried, of course. I did notice the other guys pecked at the eggs a little bit so I took them out this morning so they wouldn't do any damage. I'll keep an eye on them for a few more days