It has long been known that units under the Berserker spell sometimes behave rather strangely, specifically: they skip their turn when they could attack. This is because any unit under the effect of the "Berserker" spell can only attack the right hex (for the attacking side) / left hex (for the defending side) of two-hex units. In the screenshot, the Champion under "Berserker" can attack both the Cavalier and the enemy Champion (the distance to both units is the same). The target is chosen randomly (depends on R). So, if the Champion is chosen, the attack will be made (the right hex of the Champion is available for attack). However, if the AI chooses the Cavalier, our Champion will simply skip his turn, because the right hex of the Cavalier is not available for attack in this case (the Champion does not have enough speed). If we remove our second Champion, the skipped turns will disappear (with a probability of 1/2, the enemy Cavalier will be attacked, and even with a decent damage bonus!).
This feature can lead to a battle loop, where two units under "Berserker" remain within reach of each other, but neither unit has enough speed to strike at the "tail".