Actually, that wasn't the case. Su An'an knew in her past life that Yudu had encountered a traitor.
In her past life, after the leader of Yudu learned of the two families' plans, he decided to turn the tables and deliberately changed the shipment time to coincide with Caiying's schedule.
Caiying would annually ship a batch of pottery to West Gate around April 4th, so Yudu did not suspect anything.
Travelling from West Gate and North Fortress required them to go together for a while, separating only after passing Lin'an Port, usually resting for a night at the port.
In her past life, Yudu took advantage of that night, drugged Caiying's ship guards, and planned to move the illicit salt onto Caiying's ship, executing a 'turn the tables'; when Caiying would be inspected leaving the port, he'd be caught red-handed.
