Sometimes when you create a thread a draft is saved. So when you create another thread you see already filled text fields.
Mike said:Auto save drafts
While you are typing a message, every X seconds (currently 60), a draft of your message will be saved. If you reload the page later, your message will be automatically re-shown. Drafts will be automatically pruned over time.
Each draft is associated with a particular piece of content, such as a thread, forum, or conversations. A draft reply that you start working on in thread 1 will not be shown to thread 2 and so forth.