According to the website you're referencing, this trade slump seems to be the result of a combination of concurrent factors, including late 2020 stockpiling, firms avoiding moving goods due to fears of disruption, and low demand due to Covid-19 (in addition to the new trade barriers). Also, the slump is not seen as signaling a long-term decline: "While the headline numbers look alarming, there is likely to be a significant recovery in trade volumes in the next couple of months as freight flows have already rebounded strongly from their January lows".