Just to report something unexpected (but a very minor issue), I accidentally entered the text “arco” into the Tempo pop-over (instead of the Playing Techniques pop-over), and the tempo changes to q=72. So there may be a definition of “arco” in tempo as q=72!
I just typed “abcde” into the tempo popover, and the program set quarter = 72.
So the lesson is that apparently quarter = 72 is the default initial setting for any custom tempo you type into the popover.
That’s doesn’t bother me since it is presumably wise for the program to set some default bpm when an unknown tempo is entered.
But if you don’t specify any tempo at all you get q=120, so why 72?
No idea. 
I love it when people answer rhetorical questions 
72 quarters per minute is the default tempo value when Dorico fails to parse out any of the things you type, a fallback value. Nothing more to it than that.