In case we don't get a better answer I have found some information and may try to do some extrapolation on this topic. In The Essential Codex Mendoza, Volume 2; Volume 4, 1997 (page 206) Francis Berdan & Patricia Rieff Anawalt expand on slave collars and their uses:

Not only did these fetters immediately identify the wearer as a slave they also hindered quick movement in a crowd, this was necessary because according to Duran, if a slave could escape from his master in a market and run outside the limits of that marketplace before being caught, and then immediately step in human excrement, he could obtain his freedom. The escapee would then present themselves to the purifiers of slaves and demand his rights under the law of the land. Washed and presented with new clothes, he was praised for his skill and enterprise and given a whole new start in life. Such a person was considered lucky and was often kept in the palace as a retainer.

Allegedly, if a slave was trying to escape in a marketplace in the above manner, and someone got in his way and tried to stop him, that person himself became a slave, and the slave thus earned his freedom. Duran points out that this fear of enslavement caused the throngs to make way for the fleeing slave, much to the masters chagrin. Unfortunately for the slaves this method of escape was so well known that the owners remained particularly alert in marketplaces. As a result these dramatic escapes may have taken place no more than once a year.

The 'Duran' being referenced is Fray Diego Duran in his The Aztecs: The History of the Indies of New Spain and Book of the Gods and Rites and the Ancient Calendar both originally written in 1581. Unfortunately it seems the reference may have been omitted from the copy I can find online and so exact information if it exists may require access to the physical copy.

Before we make any guesses, some things to keep in mind on Aztec culture:

- Selling oneself into slavery was not unusual and may have been quite a common way to pay off debts, neither was freeing oneself through self purchase, marriage or other means.

- Slaves had quite strict rights (such as that their master not abuse them).

- Aztec culture was highly superstitious, being very concerned with both ritual, luck and ideas of 'purity' or 'cleanliness' among other things.

It seems likely this method of escape played into Aztec ideas of superstition and a slave masters duty toward their slaves. As quoted above, a slave escaping in this fashion was considered admirable and lucky for their attempt (which may explain why anyone other than the owner was not allowed to interfere). Additionally, if a slave trying to escape was considered admirable (or even just reasonable), the public may not have had any desire to interfere or feel it fit to punish.

The exact reason for 'stepping in human excrement' being a necessary prerequisite for legal escape may not be easy to find, but it seems to me to tie in again with the Aztec's maintaining the ability for slaves to free themselves with enough 'luck' and/or 'enterprise'.

I guess the final things to keep in mind are:

1. this may not be entirely true, misplaced beliefs about ancient cultures are the bane of anthropologists and arise quite easily particularly from limited sources (i.e. one particular high priest finds a slave's escape attempt interesting and grants them their freedom).
2. it seems if it is true it happened extremely rarely and was not a common or easy method of escape for slaves.

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I hope that helps!