I looked it up and did some reserch since this thread. and there is no answer... I cant believe that. there are somany different opinions but they are all so diferent that they cant all be true, most of them seem to agree with me, that they are top heavy and likly to roll over because the legs are curled inwards. dosnt mean their right tough.
another reason that sounded resonable to me, was another wariation of my opinion, that when they are old, or on the verge of death from poison or starvation, they are tired, and their legs cave in, only on one side, than they fall over and die on their backs.
I think that one is most likly, even though I think this lady who said thaqt may be partialy right also, maybe they roll over when their tired for a break? since they cant sit, or lie down... got to get tiring always standing.
about a quarter of the roaches I find on thier backs are still alive. so this means that maybe they roll over before they die?
lots of the people on the internet seemed like they never even seen a roach before though.
as another sidenote, about 5-10% of the dead roaches I find are not on thier backs. but kind of leaning on one side
one of the sites also pointed out thet other bugs die on their backs as well, such as flies, and loacust (cicadas)