...I have no idea what causes this and it doesn’t happen in any of the more recent IDEs. It might well be an effect of one of the other plugins I have installed (GExperts, cnPack, IDE Explorer, DDevExtensions and the Ide Fixpack).
The dzObjectInspectorFix plugin fixes that annoyance by periodically checking the width of that column and setting it to the configured minimum (Default: 150 Pixels).