So I'm looking thru my favorites and, much to my cringe, I see a few posts here and there that I've never seen before. Why are they on my faves list? I didn't click on them even. I don't have subscriptions. Yet, they're just...there. Staring menacingly back at me.
Why is tihs happening?

See this forum post bro:

https://e621.net/forum/show/192729

It's started

Why? Because somebody hacked your account, probably. Change your password, and the problem should be solved. If it isn't, then it may be a bug to report, where your favorites are reporting somebody else's, or somebody else's are getting applied to yours.