I think exactly the same as Midnight Dreary – the curse was created by Rumple and his plan all along was to find Bae. He knew Evil Queen wouldn’t hesitate to cast it once the right circumstances would occur so he’s been manipulating absolutely everyone all the time to lead him to just that. He even created the love spell to bring the magic into the SB world because he knew the curse WILL be at some point broken and he will be waiting.
If he did all that, I don’t think it would a surprise if he created the curse so that it brought everyone right to the time when Bae arrived to that world as well.
The only thing that makes me wonder is that if he knew the time, surely he would know the exact place where Bae arrived as well so why SB is where it is? He has all those maps and is ready to leave to SEARCH for him so maybe he’s not that sure where his son actually is? (that way of thinking has flows, I admit, since you can’t expect one person to stick around to one place for 28 years but if he got the time right, why not the whereabouts?)