Well, let's see...
If you check Safari
, you'll notice that there's a Press Tab to highlight each item on a webpage
checkbox. Furthermore, although it's a little ambiguous, you'll note that if you don't
have that checkbox checked, Option-Tab will produce the same result
In any event, when you're typing a reply in the text-entry box, the Submit button turns out to be the next item in the Tab queue (bonus points if you can figure out what determines the order of items in that queue). Dollars to donuts you hit an inadvertent Tab or Option-Tab while typing—thus highlighting the Submit button—then hit Return, thus effectively clicking the Submit button.
Elementary, my dear Itosis.