Bees Only Work Up?
When bees first move into an EMPTY cavity, they don't cluster at the bottom and build comb up to the top...they cluster at the top and work down. This is what bees do "naturally".
Now, if you add an empty box (undrawn foundationless frames) on top of an expanding colony, most of the time they will start building "up" so as to not leave the established broodnest...this leads to a mess, as the comb wants to flop over, so the bees reinforce it with cross comb within the first few inches...a mess. How could the bees be so stupid? Because there is nothing natural about adding empty space above the established colony...the colony will always have started at the top of the cavity, and will already have filled it.
Bees "naturally" build down because "naturally" they have already filled the comb above them.
Now, instead of an empty box placed above the established colony, you add a box of foundation or comb....this is one of the "tricks" of beekeeping. Space above the colony is filled in earnest...it is an attempt to get back to the "natural state" where the comb above is filled and the bees are working down if they are expanding. This is well demonstrated by the observations in this thread that bees "work up" faster than they "work down"....but of course working up requires that some manipulation has been done by the beekeeper to provide empty space/comb/foundation above the colony....a perfectly natural response to an unnatural situation.
A rough analogy might be what would you do if someone "harvested" your retirement account? (your long term savings is in an IRA, for the bees it's at the top of the hive). You are probably going to want to rebuild this account in earnest...forgoing the vintage PacMan machine and the Ferrari that you have been saving for in order to do so.
In short, bees will "naturally" move up if they "naturally" find foundation above the natural colony.