Syphon Overflow Tubes

I have a question for what probably is a minority of reefers on here but here goes. When topping off my tank I poured the water directly into the DT. The syphon seemed to quit syphoning. There was water still in the U tube with no bubbles but the water in the skimmer started filling and the water in the DT was on the rise. I freaked and unplug the pump when the return pump section of the sump went empty and the pump was suckin air. The return pump section in the sump is like a fail-safe it goes empty before the DT overflows. I turned the pump back on and the DT filled back up, with the U tube still full of water with no bubbles. I had to break the syphon and restart it to get it to flow again. So the question is has anyone experienced this? Anyone know the mechanics behind it? I now top off in the sump but was just wondering. Thanks for sharing your knowledge.