First thing that I found about it was the definition of ''default constructor" from programming, an example would be C++.
If I understood it well, the default constructor is one piece of program that can be called by no arguments or has defaults for arguments.
Default in such cases means "padrão" (standard), then the part of the program with a given set of arguments from wich the program had to choose.
Not a techie answer here, but let´s say the program had some arguments, say "RCL", STO, "GO TO", END, BEGIN, PAUSE, (here an HP calculator was my inspiration) and this set would be by default.
That is, there wouldn´t be others commands or operations in-between (say - addition, multiplication, squaring, derivation, etc).
These arguments would be predefined, so the user wouldn´t change then, they would be the first ones to be run before any of those operations to be performed.
Another example, just to illustrate the point, a printed dictionary comes in the alphabetic order, so if you have to find an entry/a word with the letters "A-T-T-E-S-T", you have to go to the letter A by default.
Should the dictionaries come in the reverse order you had to begin by the last letter (that would be A) by default.
So, at first, "defaulting default constructors" to me, sounds like "feeding/creating the constructor commands" That is, in the HP example, the act of feeding/creating/having ideas of wich constructors would be necessary and useful.
The engineer would think, okay, let´s just use START, STO, RCL, GO TO, and END. By doing that, he would be defaulting the defaults, defaulting the commands START, STO, RCL...etc.
Many factors would play in his decision; for instance, defaults (initial commands) with three letters would use up less space (memory) and/or would be better to memorize.
Instead of saying "escape" in some programs, using ESC would be faster and easier and would fit in the small keys of computers, etc.
Mine is just an educated guess, though, it´s what I could come up with, withouth much context.