I've seen this question posted before and don't recall seeing any of the "big guns" reply. I too have experienced this when I cut the tops off of some plants and re-plant them and I have two theories that may occurr separately or be combined to cause the issue:
1. The stem is damaged during the cutting/replanting stage e.g. use sharp scissors (no pinching) to cut the stems of larger diameter stem plants and dont crush the stem when you re-plant the top.
2. Make sure the substrate isn't stagnant in the spot the new plant is placed. (aerate the substrate with fingers prior to planting)

Theories only, I have no cause-effect data to back this up.

Aaron