It is just a setting.

If you start a thread or click Use Full Editor the default setting is to put that thread on your followed thread list. Every time a post is made in that thread you'll receive an email
IF you have your preferences set to "Send me an email when a new post to a forum, thread, or by a user within my Follow Lists is created". If you don't want to receive those emails you can either uncheck the "Add this thread to my Followed Threads" box and/or edit your preferences and uncheck the box next to the setting I mentioned above.