Hi Sarah,
I am sure there are more experts than me around but I do know that it is not odd behaviour it is an instinctive reaction, same with humans. All animals wil experience instinctive sexual behaviour. Some babies aged around 12 - 18 months will get onto all fours and look as though they are shagging the floor, my next door neighbours did and whilst she got upset about it, it was short lived and did not last long.
Whatever your pussies are going through I am sure that it is natural and not odd and in any case I presume that you will be getting them neutred in the future and so this behaviour will probably stop.