I've seen edits from trusted users who didn't alter the individual question tags, but switched their order. Is this done by a site rule, or for SEO? Is the idea to put priority tags first? Has this been covered somewhere?
It's automatic. The site orders tags by their popularity (number of questions tagged with a tag), regardless of what order you enter them in. If the popularity of the tags changed after the last edit, even if the editor does not touch the tags the tags will still be show as reordered on the revision diff.
I'd like to see one such edit because I'm pretty sure that's not actually possible.
Tags are automatically ordered from left to right by role (required/mod or normal), then popularity (AFAIK that's just how many questions use it).