Everyone's suggesting captain Lorca is originally from the mirror universe. There's some problems with this line of thought because it's just too convenient plot-wise for this to be the case - the fact they got into the mirror universe cannot be blamed on Lorca without destroying the entire plot about the spores so far, and so the coincidence is just too much to bear - but I guess it makes sense tonally speaking.
Ok, so they went there. Now they have to explain how on earth he pulled it off, because it just seems incredibly ridiculous plot-wise. There are so many moments that do not fit into this arc, that I'm having trouble fitting it together. OTOH, there are others who do now, like for instance him keeping the blaster near himself while he made love to Georgiou.