| Disclaimer: I speak based only on my own observations...
Popcorn appears to be low-glycemic. I cover it primarily with NPH, which has a 5.5-hr completion time in me.
Just last night, I severely underestimated my popcorn intake. Two-hour pp was fine; 3hr pp was 177 (corrected with aim for 100); and I awoke during the night (waaay post-prandial) with 170.
My guess is that the slow CHO release triggers your insulin release... then gives the insulin a chance to do its job.
One of my favorite ultra-low glycemic foods: Tinkyada whole-grain brown rice spaghetti. You might try it to see if it has a similar effect. | 
