The answer to your question is, yes I have noticed this as well, dumping back up a level. I believe there is a setting for action to be taken after a post. It may be on your profile management - have a look for it.
Well, I found it, sort of. If you click "additional options" just above where the post/preview/spell check buttons are, you get a checkbox for "Return to this topic". Nothing in my profile that I've found thus far, however.
Edited to add: I tried the checkbox, and it works once for that post only. I wondered if maybe it would remember and keep doing it, but no such luck.
Edited yet again to add more thusly: Ah, it remembers that I checked the box for THIS post, and keeps putting me back into the thread. Of course, I'm just editing the same post over and over, so maybe that's not surprising.
Ah, exploration!