Good wag, Answer
As Dan said. when you have warm glue on top and there is seepage from similarly heated glue from underneath through the veneer itself, it moves the glue away from collecting and cooling in the middle of the piece. The center would be exactly where you don't want pooled and cooling glue. By moving from the center out the glue has a place where it can be scraped away at the edge and where the gunk often is trimmed away anyway from the edge overlap after the glue bonds.
Taken from Bo Riley, Hot Glue Veneering
Upon completion of the case assembly, the large walnut panels are the next to be veneered. At this point I brush with thinned glue all of the surfaces that are left to be veneered. After the glue has dried place an oversized piece of walnut veneer in the correct location and trace lightly with a pencil the correct location of the walnut. Apply hot glue to the marked surface quickly and well overlapping the lines. Put the veneer in place and brush glue on top of the veneer. Yes. on top! This acts as a much needed lubricant, equalizes moisture content for both sides of the veneer which reduces cracking, buckling and curling, and fills and seals the grain which makes subsequent finishing easier. Quickly rub the piece with the veneer hammer first with the grain then diagonally in all directions working from the center outward. If the hammer starts to drag too much you may either use more glue, or if the pot is out of reach, saliva does very well. The entire process takes about 5 minutes for each panel. Wait no more than 30-60 minutes before trimming the excess walnut from the panel. It should peel off like stiff leather. To wait too long is to find out how tough hot glue really is.
Some helpful hints:
Keep the veneer hammer warm;
Always keep warm water available as many uses will arise;
Be sure stringing joints are tight from top to bottom. Unlike carpenter's joints, stringing joints need to be reduced to the level of the surrounding veneer. If the point of contact is not tight all the way to the bottom, an ever widening gap will appear as the stringing is made flush with the veneer;
Stringing can be held in place by pins inserted beside the stringing bent downward upon the stringing;
Large pieces of veneer may be heated with an iron prior to gluing;
Use an old dull chisel to scrape warm glue away.