When Amon flew high up in the sky to get behind the Royal Puppet Army, Wai Kung and what's left of the Grand Militia were on their way to get behind the First Demon Army. They were taking a long, roundabout path to avoid the enemy's defensive line. If they could do that, they could disrupt the enemy's Yuanxu Cannons and give the Royal Puppet Army a better chance at victory.
They stopped when they saw some enemies trying to cross from above using spirit beasts. One of their people used Eagle Eye and identified Amon among the enemies riding those spirit beasts.
Wai Kung didn't believe the enemy commander would go behind the enemy line with so few supports without a good reason. Additionally, Wai Ling saw Amon as one of the main targets for her revenge. Hence, they turned around and went where Amon appeared to be heading.
