Guy has already noted some of it. In general, there is groundwater flow into the area around the pumping plant. Probably more important on the Sakakawea side. Anyway, like Guy mentioned, slight changes in water temps keeps the water moving, even if it's not that perceptible to the fisherman. Those humps and areas between islands you noted that have historically had thin ice, will always be thinner than out in the flat bottom parts of the lake. The reason for that is the smaller cross-sectional area, as the water moves up and over the saddle between a pair of humps/islands, it has to move faster to get the same volume of water through the smaller cross sectional area.
It can be tough to pick out dangerous areas when on the ice, but when looking at Google Earth or NDGFs lake maps, you can often see areas that are more prone to it. Like is said, look for the saddles and humps under the water next to deeper areas.