
Bitches in season can get grumpy, and with the raging hormones will indulge in mounting behaviour, which the other bitch may not appreciate, or may take as a challenge to her superior role. This can be a time when a tiff can become nasty if you do not intervene.
I once had 3 bitches in season together within days of each other, and the behaviour was very flighty and could easily have got out of hand. I did quite often have to bellow at them to settle down, and not annoy each other.