Difficult to say, but you may also want to check the Wowza logs at around the same time to see if any relevant event was captured, [wowza-install-dir]/logs/wowzastreamingengine_access.log. If you feel that it may point to a Wowza issue then please do raise a ticket with
firstname.lastname@example.org (quote this forum post) and we'll be in touch very quickly.
Maybe unrelated, but I did read elsewhere that some people were having more luck with the
libav fork of ffmpeg in combination with
Black Magic encoders. Certainly something to also keep an eye on even if it ends up not related to the current issue you're experiencing.