Strange Artillery behaviour

Today in my game was a situation I have never met before. I executed an attack with my Artillery and Rocket Artillery stack on two Infantry units. After the first hit the infantries immediately split and one of them started to move toward my battery. The battery cannons stick to the still unit while another one kept moving. Close to the end of the attack round I manually switched attack focus to the moving unit and it was killed by that attack. The battery automatically returned to still unit and I was preparing to wait for another half an hour for the next round but attack timer started to count not 30 min but about one minute after which the cannons hit the poor unit to death. Surprisingly pleasant but still strange.

Is that a normal behaviour? Bug? Never happened to me before. Anybody could explain?