Another piece of machinery somehow relating to it suddenly blew a fuse just seconds before the transport. At which point our heroes immediately informed the transporter room. A tad too late, of course, but c'est la vie. I don't really see the supposed incompetence here.
My impression is that the "bad luck" wasn´t that they had already started beaming, but actually that the materialisation process had already begun when "the fuse blew". If the materialisation had not been started at that point, then the matter stream would still have been in the buffer and could have just been sent back to the platform on earth. But once the materialisation had started, there was no way to "just send them back safely"....