In the same way, you could factor 6 as (-2)(-3). It's not wrong, just different.
I recommend, for several reasons, always factoring so that the leading terms are positive. This preference makes many tasks easier; for instance, it avoids problems in factoring by grouping. It is not the only valid answer, but is a good habit.
As to why they do it -- ask them! I'd be interested to know. I suppose somehow -1 catches their attention first. Or maybe they just like being different!