There are a number of factors that could be contributing to your issue. The three most common we would look in to first, are dough hydration, baking, and proofing or fermentation.
If the dough is too firm, too dry, it can cause it to contract and concave as you describe. If you are using a flour that is too strong, or adding more gluten than needed, it will exasperate this even more.
Baking can also be cause or contributor. Over baking can cause this, and part of that may be not getting the buns off the pans to cool soon enough. As long as they sit on the hot pan the keep baking.
Green dough that has not had sufficient bulk fermentation, or final proof, can often exhibit this characteristic, or contribute to it.
We hope this helps you trouble shoot further.