The interpretation I have found in general usage is that Ra had grown visibly old, which was suspicious to the human race, who did not even know what death was (as hinted at in your Roland Mastaff quote). Based on this, humankind had lost confidence in Ra's fitness to reign, and essentially they were staging a coup d'état. The whole business is dealt with very tersely in the cursory prelude with which the Book of the Heavenly Cow starts off:
Once it came to pass, under the Majesty of Ra, who brought himself
into being, that when he had been in the kingship over mankind and the
gods combined, mankind proceeded to contrive a plot against the person
of Ra now that His Majesty (Life! Prosperity! Health!) had grown
old, his bones being of silver, his flesh of gold, and his hair of
genuine lapis lazuli. His Majesty became aware of the plot being
contrived against him by mankind.
In her 1988 book Egyptian Legends and Stories, M.V. Seton-Williams frames the incident in the context of humanity's refusal to continue supplying the god with due honours in the form of sacrifices. In the introduction to her own retelling of the story, she says (on p. 22)1 that:
In the beginning mankind was grateful to Re for the benefits that he
had conferred upon them, and they worshipped him and supplied all the
things necessary for his service. After a while, however, they became
tired of doing this and claimed that Re had grown old, and had no need
of further offerings, so supplies for the temples diminished.
Joshua J. Mark, in the "Book of the Heavenly Cow" article2 of the Ancient History Encyclopedia website, surmises that:
The story begins with humans plotting against Ra because he has grown
old and they feel he can no longer manage the affairs of the world. Ra
hears of their scheme to overthrow him and calls the other gods to
council to ask their advice on how to proceed.
Roger Lancelyn Green's conclusion, per his 1970 book Tales of Ancient Egypt, is that Ra was indeed enfeebled by his age, so that "he could no longer rule well over the people of Egypt" (p. 4).3 He blends his rendition of the story into Ra's well-known role as the opponent of Apep [Apophis], going on (on the same page) to imply that part of humanity's motive was fear that the god was no longer capable of fighting this "Dragon of Evil who had grown out of the evil vapours in the darkness of the night and sought to devour all that was good and bright and kissed by the sun."
This is a spot of narrative harmonisation on Green's part, as his book is after all creative fiction targeted at children. In the actual Book of the Heavenly Cow source material, Apep is mentioned merely one in time, in passing, as having a soul that "is in the Eastern Mountain," as opposed to the soul of Ra, which "is in magic throughout the world."
An interesting vindication of humankind's position, if it is correct to conclude that the threatened coup was on account of the divine king's perceived weakness, is that Ra seems to confirm their suspicion, in a bit of an ironic twist in the end. In the aftermath of Sekhmet's rampage by which she nearly annihilates the human race, Ra complains to Nun of his heavy-heartedness, and, smitten with weariness and pain, he admits: "My limbs are feeble as in the First Time. I will not return until another (cycle) overtakes me."
The story then goes on to explain the origin of the Celestial Cow as the vehicle for Ra to basically escape his ungrateful subjects on earth and depart into the heavens. As Joshua J. Mark puts it, "There are, in fact, a number of etiological elements to the myth which explain a wide range of traditions from why maidservants brew beer to the creation of the afterlife of the Field of Reeds."
Nadine Guilhou's UCLA Encyclopedia of Egyptology article "The Myth of the Heavenly Cow"4 explains that this story
displays how the gods share power and tasks after the departure
of the sun god. Shu becomes the support of the Heavenly Cow,
separating the sky from the earth; Geb is now responsible,
together with Osiris, for guarding the earth—in particular the
mounds and the snakes that inhabit them. This scenario can be
understood as a “spatial distribution” of power. Moreover, the
installation of the god Thoth as both vizier and moon,
substituting for the sun god in the sky each night, can refer
additionally to a “temporal distribution” of power.
1. Of the 1999 Barnes & Noble Books, New York publication
2. "[P]ublished on 22 August 2019", viewed 5th August 2020
3. 2016 edition, Puffin Books: An imprint of Penguin Random House LLC, New York
4. Jacco Dieleman and Willeke Wendrich (eds.), 1004 Version 1, September 2010