a) it’s quite common, so a lot of people have encountered it

b) feet are rather dirty, so it evokes disgust in those that don’t have it

This creates an environment in which people can commiserate in their disgust with others by means of insult. This creates a meme that people will take on as a belief to join the in-group / avoid being targeted by the majority.

It’s the same as a lot of arbitrary hate, honestly.

It is conceivable that the glue was put there for another purpose, and the pigeons simply landed in it. That’s just not likely in this case