I took no steps towards this. I can't see any hashtags from my end.
I don't use mastodon but I vaguely recall something about a bot that would add hashtags to lemmy posts to get them visibility on there. It was opt-in at some level but not per-user or per-post. Maybe the community moderator? Or the instance admin had to install something?
I clicked through from your comment and I found my profile on your instance, https://masto.ai/@[email protected]. I see one other post has been tagged with #piracy which I also didn't take any active steps towards. Maybe if you use this method to look at a more active user you can deduce where it's coming from.
However, I know that magazines on Mbin (equivalent to a community on Lemmy) come with an option to automatically tag all posts made in the magazine with a hashtag. I used to do this with a magazine I modded till the instance I was on (kbin.run) died, taking the community with it.
It's possible the mod of ObsidianMD did the same thing with the obsidianmd hashtag. However, I see that ObsidianMD is a lemmy.world community. I'll need to look into this more.