The cause is pure physics. First, cut the end of the tube off at a 45 degree angle. Second, provide a small secondary drain or a Tee near the end of the hose that will still drain where you want it but acts as a pressure release as well as an overflow at the end of the drain tube in the sump. Then, make sure the end of your 45 cut is not all the way submerged when the sump fills up during the simulated power outage. If you have a U shape, where the U has a bottom like a sink pee trap, you can capture all kinds of sediment and sponges in there that will plug the line or hold air locks. The U should be more like on the back or a C. If you have extra plumbing, take time to adjust or cut it now and avoid this later when you are away for a couple days. That would make a mess and could melt down a pump, depending on the type you use. If you have a vertical drain line, small holes in top (if you have a silencer) and end help avoid this combined with the 45 end cut and T, if necessary and not below water level all the way. You could have the 45 cut below water level to limit splashing, so long as you have a secondary drain above the water line. Thats where the Tee comes in and it will prevent an air lock. Of course it must be in the sump or you can still have a mess.