That is indeed a sign of overproofing. The relaxing is from the gluten overstretching during the proof phase, not as common in these cold months, but that is what is happening.
If you are having a tough time knowing when they are ready to load, consider “learning” about your loaves by placing them in the oven in staggard intervals. Put the first one in at 45-50 minutes, the next one at 60-70 minutes and so on. You will be able to see the difference.