I have seen this behavior with female cats that are unfixed during their mating cycle. If your cats are not yet fixed, you should get them done as soon as possible. Also, you should be aware that females that are ready to mate will fight with other females if they smell a full male anywhere, including outside. Scents can also be transferred to you or guests via something that a male has marked and you brushed against it outside without knowing it was there. Your cats will react to the scent even if you don't smell it.