That I don't know either. People have a theory they have some sort of machine. I talked to one of the scientists about it, seems they can't control who, where or when they pull someone in.

Thatch? {Why would--oh. Right.} Nah, I haven't seen any of you since I got here. Croc was here for a time and Doflamingo too.

They do like to treat us like yo-yo's though. Luffy and some of his crew have come and gone several times. Every time they come back something's different too. Time's changed or they lost their memories of this place all together.