This surprise attack that sayuki received from behind reminds me a lot of the ability of the site administrator who had used the body and magical ability of yatsumura alongside the other administrator named 'juuroku'.
This would explain how the site administrator named as 'go' mysteriously disappeared without a trace since it seemed that the screen had frozen for a while and sayuki had been cut in half by a similar magical ability.
Remembering that the last appearance of the site administrators 'juuroku' and 'ni' had been in chapter eighty-three and the magical ability of the axe of juuroku was also related to cutting things - they proved to be strong enough to kill six magical girls of the team at once and we still don't know what they were doing after the showdown.
So maybe the author was waiting for the right moment for them to return since 'ni' is the most dangerous administrator right now as she manages to paralyze time.
However, maybe it's just the hidden magical ability of the demon blade that ended up killing sayuki.
This only shows how they may not have the slightest chance against the site administrators, each one seems more powerful than the other and we do not know how much they are capable.
Anyway, sayuki have to use a good part of her lifespan trying to eliminate just one site administrator.
It's only a theory, sorry for write too much - probaly it's just bullshit.