When Immortals clash, mortals suffer.
Su Chengyu never expected Master Chang Chun to visit in person, let alone to start fighting with Lord Chou upon arrival, leaving him completely unsure of his place in the situation.
Although trembling might be an overstatement, he did indeed feel somewhat superfluous standing there.
"Teacher, I'll excuse myself for a moment while you and Master Chang Chun catch up," he said.
"Catch up? What do I have to catch up on with him? Both the Heaven Punishment Department and the Imperial Observatory share the character 'heaven' in their names, but do you think we're really familiar with each other? Not by a long shot," Lord Chou retorted.
The arrival of Master Chang Chun had clearly made Lord Chou much more active than usual.
Perhaps there really wasn't much interaction between the two departments, but the two had certainly had private contact.