Following Bob Jones’ tip to search Herodotus on the previous question about 'flesh cutting' I found a very interesting article that seems to explain the circular hair cutting. It seems the "secular source" for much of these pagan rites come from Herodotus.
According to an article by Avram Yoehoshua, Herodotus said:
The Arabians acknowledge no other gods than Bacchus and Urania (i.e. the Queen of Heaven), and they say that their hair was cut in the same manner as Bacchus's is cut; now, they cut it in a circular form, shaving it around the temples."
Avram argues along many lines, as he seems to be quite disturbed over the 'kippa' as something very personal to his Jewish roots, but it is all very fascinating:
There are places in Scripture where the Lord tells us not to shave our heads in the form of a circle (because this is originally what all the priests of Baal, Bacchus, Tamuz, Apollo, Jupiter, Dagon, etc. did), to signify their allegiance to the sun god.
As the sun-god was the great lamented god, and had his hair cut in a circular form, and the priests who lamented him had their hair cut in a similar manner, so in different countries those who lamented the dead and cut off their hair in honour of them, cut it in a circular form.
There were traces of that in Greece, as appears from the Electra of Sophocles (line 52, pp. 108, 109); and Herodotus particularly refers to it as practiced among the Scythians when giving an account of a royal funeral among that people.
"The body", says he, "is enclosed in wax. They then place it on a carriage, and remove it to another district, where the persons who receive it, like the Royal Scythians, cut off a part of their ear, shave their heads in a circular form," &c. - (Hist., lib. iv. cap. 71, p. 279.)"
Interestingly, Avram also says the 'round form', as can be seen from original Babylonian statues of their gods, is where our modern day 'halo', Jewish 'kipa' as well as some historical 'shaven crown or patch worn by monks and other clerics' can be traced.