V.T.
If you haven't changed the flour you are using (some have less gluten and don't form the bonds to give the bread structure) it could be a few things that are likely either related to the particulars of the way heat circulates in your oven or the humidity in your home.
Try giving your breads a few more minutes (10 or even more) in the oven to fully cook the center of the bread and reduce the moisture content. If it retains too much moisture then the bread will fall as it cools. The outside might get a bit crisper, but that will soften once it's out of the oven and the moisture balances in the loaf.
Also, if you don't turn your breads in the oven, give that a try. I'm baking in a relatively new ('90's) oven that just doesn't heat as evenly as my old 1950's model and find that I do need to turn about 1/3 way into baking for even cooking.
If those approaches don't solve the problem, see how the bread reacts if you let it cool up to 20-mins in the loaf pan. If its still falling, then there's definitely too much moisture in the loaf. Another solution is to make slightly smaller loaves that bake more evenly -- this is a viable solution when you're battling with high humidity and uneven baking temperatures.
Good luck!