I noticed that one of the tags I used in my question has been removed. Are there cases where tags automatically removed?
Tags that are not used from any question are deleted every 24 hours from a background process.
Tags that are used from a single question, and that have been created more than 6 months ago are automatically removed. When this happens, and the question used only a tag, the removed tag is replaced from untagged. See What is the "untagged" tag? for more information.