Patrol wheel came off..reasons?

Morning all,
My work entails the use of a Patrol which to date has only 8,000 kms on it. Whilst driving it yesterday I noticed a slight temor through the steering which I put down to a wheel being out of balance. The day prior it had been through some solid mud and is often the case, it collects and drys inside the rim, causing an inbalance. As we were on our way somewhere, decided we would clean properly once we arrived.
Didn't happen though. Withing a couple of kms, felt the feedback increase through a sharp corner and continue once I straightened up. Decided to pull over and whilst slowing down, an almighty crunch as the wheel came off and the Patrol dropped down onto the disc. Luckily we were down to 20kms at the time and stopped without incident.
Checked the studs which weren't stripped or snapped and appears the nuts have simply worked loose. Car was towed to Nissan for inspection and a safety check. Enquiried with our fleet management and were told it is the third Patrol to have this issue in the last few months. The pevious 2, Nissan dismissed as incorrectly tightened nuts, 'not withing the specified torque range'.
Wondering if there are more who have had this proble. All Patrols are curent models with the alloy rims. We're looking at requesting the steel rims as believe the alloys might be the problems. Your thoughts?