Not cooking them long enough......

If the sink AFTER taking them out of the oven, I believe it's much more likely that you have under baked rather than over proofed. I have over proofed many times, when I was shaping rolls faster than my ovens could bake them, and the only time the center fell was when I rushed the rolls in the oven and didn't allow them to really get done.
another possibility could be not allowing them to rise enough before baking and you got your oven spring only on the outside of the roll, or perhaps you had refrigerated your dough and it did not warm up enough to rise properly...or maybe you're baking your rolls by the color and not by the time in the oven, and they are getting too brown before the center is done and they drop when you take them out.
Whatever....if they drop after taking them out....they were not baked long enough.

Thank you so much ladies. I think both of you have figured it out. I probably do over proof before popping in the oven and I do cook by the color of them and not the time. I will be a bit less generous with the proofing and a lot more generous with the cooking.

Thank you for all the great help, I really appreciate it. Mary

This happens to me sometimes if my filling is to wet. I usually brush melted butter over the dough and then I squirt some honey over it and then throw my dry ingredients down. Sometimes if I am too heavy handed with the butter or honey I find they sink a bit but if I am careful and watch myself I don't have this issue.

Alton Brown recommends baking cinnamon rolls until they reach an internal temperature of 190°F. I generally do check the internal temperature of any breads that I bake, just to make sure. It's been a while since I've made cinnamon rolls, but maybe I should start making them again. I frequently bake them in muffin pans.

I believe there is no such thing as too much butter or honey on sweet rolls!!
If your rolls are getting too brown, lay a piece of foil over the top until they finish baking....and Lars has the right idea...take the temperature of the center of the rolls in the center of the pan. Alton Brown may like 190 but I find that a sweet dough is better at 200 degrees....more stable.

Not too brown at all... Just find if I over do it on those things the center stays a lot softer and sinks down a bit and thought maybe that could be part of her problem with sinking buns.

An old baker taught me to make all sorts of breads and one of her tips for anything rolled was to not put any filling near one long edge, begin with a tight roll from this side so there is no filling in this center roll. It works for me, but I will remember to take their temperature before pulling rolls from the oven.

Hmm...I always want lots of filling in the center and leave the trailing edge fairly clean....so it seals well.