It can also happen when a virgin queen has emerged and the exit “flap” at the tip is resealed by workers. You think is this ever going to emerge? Next thing eggs everywhere, she actually came out at the normal time. If you open them up you quite often find a worker entombed within. You can usually tell if they have been resealed by scratching slightly at the edge of the tip, the top will reopen as a flap rather than a ragged hole.
Good to hear you opened it to have a look. Can tell you quite lot about what is or has been going on, lot of people just cut or crush them out and miss the opportunity.
As Chris rightly says, sometimes they prove to be duds or likely problems, which is difficult to know from the outside but there can be clues such as being overlong and thin, a smooth exterior or short and stumpy, sometimes little bigger than a capped drone cell or bent as it encounters an obstruction such as the bottom bars.
“Pick a good queen cell” is often more problematic in practice than it suggests. Perhaps “the best available all things considered” is a rather less punchy but more honest line.