Well, if it were flipped the other way wouldn't there be a risk of the components compressing the solder thereby spreading it and making incidental connections?
It wouldn't work that way. The reflowed solder is significantly more dense than the attached parts. The parts would literally float on the molten solder and would center themselves on the pads as a result of surface tension. And the solder itself is constrained by that same surface tension to stay directly over the metal pad on the board.
As fas as parts falling off, I think only the very heaviest parts (coils, stuff with heatsinks attached) are at risk of falling off, as even molten solder is "sticky" enough (surface tension again) to retain most SMT components against the force of gravity.
This problem was caused by Windows, which was created by Microsoft Corporation.