I wear minimal shoes for all my running, vibrams/pumps/windsurfing shoes and I have noticed a similar effect - sometimes on runs of marathon length or longer the official photgraphs towards the end of events have revealed clear evidence of heel strikes creeping back in to the equation.
I have always just put this down to tiredness and as I was a heel striker for many years I think it is easy to revert when fatigue means you lose focus on your stride. Now if I spot it, I just concentrate and enjoy getting back into feeling my forefoot strike first.
I think it's just the body finding the path of least resistance in the same way as you might slouch towards the end of a long walk.
I'm really interested in this, being local to the route and enjoying a longer challenge, but running 2 laps puts me off as variety is a big help for distraction of the mind in the closing stages of races over this sort of distance and second £30 does not favourably compare to the £15 bargain with t-shirt and bounteous checkpoints that is the Round Rotherham.
denotes Subscriber-only content | 
Stickit2
