Can I ask if this is normal and if I have somehow violated a community rule?
Moderation is done by one's own hand on Hubski. The ability following, un-following, or muting tags extends to users alongside a couple more features. That said, I would hope others take agency into their own hands when it comes to curating their feed and what they want to get out of the site. Is it normal? It can happen. Did you violate any community rule? Nope, someone merely thought it wasn't worth the time for whatever the reason may be.