Info from:
A Nabataean and Roman Domestic Area at the Red Sea Port of Aila
Alexandra Retzleff
Bulletin of the American Schools of Oriental Research No. 331 (Aug., 2003),pp. 45-65
jstor.org/stable/1357759…
Pre-Islamic and islamic burial
Info from:
A Preliminary Report on the Cemeteries of Bir Madhkur
Megan A. Perry
Bulletin of the American Schools of Oriental Research No. 346 (May, 2007),pp. 79-93
jstor.org/stable/2506701…

Although laying to rest the body on its side is not typically considered a Nabatean practice, there is nonetheless apparent evidence of this ritual in the pre-Islamic period.
We have seen that Mt Sinai is the only contemporary religious site along this direction.
Did early Muslims adopt these burial customs from the Nabatean people living in the area?
Or does this represent a transitional period between Nabatean and post-Muawiyah I burial practices?