What causes components to "jump" during reflow soldering?
The flux in solder-paste is indeed hygroscopic.

I have experienced this same problem when assembling prototype boards with old paste. Over time, the paste seems to accumulate moisture, and pop more and more vigorously.

The only solution I have found is to buy new paste. Refrigerating it does seem to extend the shelf-life, but it still goes bad.



It may be possible to gradually warm the board with solder-paste and components to ~100°c and then holt it at that temperature for a while (maybe half an hour?), to try to drive out moisture, and then go directly to the actual reflow heat without letting the board cool. This is how they deal with components that are moisture sensitive, I just don't know if it would work for the solderpaste too. 

Really, solder paste is pretty cheap, just buying new paste seems like an easier solution.