Thanks! I'm speculating one of the plugin modules found on the Modules tab on the Orbiter Launchpad just before you start any of the scenarios (the settings where you could activate TransX, OrbiterSound, ScenarioEditor, etc.) are causing the problem.
If you have Attachment Manager installed and activated, deactivate it first, but like I said it only causes the Soyuz rocket to explode on the pad and (may) not be the one causing it. If not, randomly deactivate any of the plugin modules before you start any launch scenario.
If the bug still occurs, you can ask help to the author of the Soyuz rocket, igel, by posting it
here