I don't think you're doing anything *wrong*
per se by posting to old threads... but I do understand people complaining about it. For many of us, threads this old are essentially long-finished conversations, and having them brought back out to the front page with the attendant assumption that they are new conversations, can give you a sort of cognitive whiplash. It's disorienting and wierd to suddenly find yourself back in the middle of a conversation you 'finished' several months back.
In a strange way, it's like fighting with your wife - stuff that's dead & gone *always* seems to find a way to come back to hurt you when you least expect it.