We're thinking about this very strategically. Is it possible the Borg think algorithmically, and so a lot of their decisions don't make sense from an individual/human perspective? Maybe that specific Cube assimilated a time-travel device on its way to Earth and instead of, say, considering the ramifications of time-travel, it simply attached it to a Sphere and labelled it: "Only use in case of extreme emergency." Then, in an extreme emergency like the imminent failure of the Cube's mission (assimilate the Earth), it triggered the device as a self-protection mechanism, but didn't really 'think' about the ramifications of that decision beyond the immediate, programmed mission parameters of 'protect the ship'.