Cake – why might the pineapple on the bottom of the pineapple-upside-down-cake float into the batter

Yellow cake made from scratch is given its color and flavor from egg yolks. A cake mix from a box, although you didn't add any eggs to it, might already have egg product in it. If it was one of the major brands of cake mix then it will likely have additives such as flavorings and food color to give it its distinctive flavor and color. In the homemade realm a white cake is made with only egg whites and a yellow cake is made with both whites and yolk.

Instead of worrying about how well the pineapple is sticking to the cake, I'd try different pan preparations to make sure the pineapple doesn't fix itself to the pan.

Looking at the recipe that Holly linked to -- it says specifically not to stir the butter and sugar together, which I assume would give you a better separation as there's butter against the pan, and you're doing it in a pre-heated pan, which helps to prevent sticking (but normally isn't done for most other cakes, as it'll cause them to dome on top)

If you're not making the caramel in the same pan (and so you wouldn't have that layer of melted butter), I'd be inclined to use a parchment circle with another layer of fat over it, so after you invert, you'd then have to remove the parchment (which shouldn't be too bad if done quickly, before the caramel sets)