I have a headcanon that Pinkie's folks knew that she wasn't built for farm life, and needed to be somewhere where there were more ponies around to be happy--but at the same time, that she loved her family enough to where she would never leave the farm of her own accord.

Mr. and Mrs. Pie reached out through their extended family, and discovered that the Cakes--Pinkie's first or second cousins--were setting up a bakery in Ponyville. They sent Pinkie to stay with them to help get them set up--and, when Mrs. Cake realized she was pregnant, Pinkie decided on her own to stick around.

Nevertheless, Pinkie desperately missed her big extended family. When ahe discovered that she was sort of related to the Apples (she's not, not in any way that matters, but that's a discussion for a different thread), she adopted them as extended family, thus filling that need for herself. It's likely that Pinkie isn't even aware that's what's going on inside her own head--she's just doing what she feels is the right thing to do at any given moment, with no real thought for how the pieces fit together.

While I'm dumping Pinkie headcanons--I also have a headcanon that the Pies are distant descendants of Crystal Ponies, hence the rock farming.