Thanks for all the replies.
I'm expecting a call from Bell today, so will post the reply. The first guy I spoke to over there suggested the change in pressure with altitude. However the question on that is that if we had gone higher, would it have dropped below the red line? If we were only at 9000 MSL and near the limit how do people get upto 14-15,000? The temperature up there was well above freezing. She's due for a 100 hour in about 35 hours.
I personally think there is more to it than air pressure change, but time will tell!