Of course you are correct - we are not talking 'Piper Alpha'-type major incident - but in an environment where everything is meant to be so closely monitored, controlled and carefully planned? For an aircraft to turn up on a rig uninvited is one thing, but to continue to land I would consider a 'major incident' in pilot CRM-aspect assessment of risk and breakdown of procedures.
My apologies, I did not want you to get too excited ;-)
I suspect that over-familiarity and slacking of procedures can be held responsible here - waiver following: This is merely a rumour forum and I have not the foggiest why it happened as I am fortunate to be many miles away from the scene and happy to take a calculated guess as no injuries, just pride :-)