I have seen many animals do this...cats and dogs...over the years. I have not always been able to detect anything on 'the spot' but other times whatever they wallowed in smelled horrible but was indistinguishable as to what it started out as...some had to have baths to be indoors with me. I've seen many explanations given.
I have recently seen Brody do this, but since I was right there with him, I stopped him and could not find anything there, or detect a scent on either him or the ground. I moved both Yorkies away from that spot, although Mia did not get to rub the spot...she was very interested in whatever was/had been there...and I am certain they 'discussed' the even in greater detail later.
I did have one female kitty 'pay respects' to a fallen fellow kitty in a similar fashion, however, by wallowing on her fresh grave. On that occasion, she seemed to completely understand what had happened, and her actions, though hard to adequately describe, seemed very respectful of our fallen, well-loved kitty.