Sometimes when waiting for a team up, I tell crew to finish tasks, or pets to train, or eat snacks. I found that when the confirm screen popped up, I was in the middle of clicking something else, and accidentally declined. It can be easy to do unintentionally. Maybe this is happening to others too. Now, I try to keep an eye on how much time is on the countdown, leaving the other screens earlier, and try not to rush clicking through the pet stuff, in case the confirm screen appears early.
Sometimes, the other player can be waiting for a while for a team and when the confirm screen appears, they may have gone AFK, or even just looked away for the 30 seconds the screen is up. I figure sometimes a parent, somewhere, is also saying to a child, " I said log off NOW."

Willowydream on Feb 15, 2018 wrote:

I have had that happen sporadically rather than regularly. I wish I knew what caused it.

I have noticed that occasionally, the first name in the team up disappears and it starts to look like I created the team up, so possibly Calamity Finch is onto something that the other pirate either logs off or times out, leaving the ledger in a bit of a quandary as to what to do with us who were waiting.

In addition to the other causes I mentioned, I suspect that sometimes a person changed their mind and canceled their participation in the team up, for whatever the reason ( bathroom, out of time, decided to do something else, friend called for help etc.). As someone who likes to help people who have no one else teaming up with them, I can say, when they leave team up for whatever cause, the one who joined through the ledger moves over on the list, as if they are the one who started the team up. If you ever joined to help and wondered, why is this max pirate seeking help in a Skull Island dungeon, that is probably what happened.
I know it can be frustrating when what happened to sunshine343u happens, but remember that there are many good and accidental reasons someone might not complete the team up process.