What Causes This?

Were you contrasting table sugar vs. corn sugar? I think his distinction would be too small to be the create a problem of the magnitude I think the OP described.

 

Simply asking a question to obtain more information.

I also posted my process for 'mixing' the sugar solution with the beer since I have a suspicion similar to @Mothergoose03 posted above that it might be an uneven mixing issue.

Cheers!

 

I've resigned to the conclusion that some home brews just do this. If I'm careful, they don't necessarily erupt out of the bottle, but they'll still create a monster, non-dissipating foam once I've poured into the glass. I've spoken with other homebrewers who have this inexplicably happen on occasion as well. In my experience it doesn't seem to correlate with a highly carbonated beer, nor with an infection (at least not one that I could taste).

FWIW, every bottle of Orval that I've ever had has done this also, so I take comfort in the fact that I'm with good company.