Why are many truckers in Canada upset about the proposed mandates?

There might be different reasons, but one reason is that truckers were mandated to wear masks even when they were driving alone in their cabin – often for many hours. 

Most severe illnesses and deaths that are attributed to COVID-19 have serious comorbidities. The illnesses may be influenced by the infection, but less than ten percent can be attributed directly to the virus itself. 

https://justidag.info/en/covid-updates/what-are-the-causes-of-death-in-the-pandemic/?search_highlighter=CDC

Of the illnesses that are directly instigated or triggered by the virus, bacterial pneumonia is probably the most prevalent cause of severe illness and death. If you are infected and wear a mask for a long time there is a risk that you aggravate your illness by constantly reinhaling your own virus and bacteria instead of getting rid of them by breathing, coughing, sneezing, talking, laughing or singing. 

http://justidag.info/face-masks-are-dangerous-sometimes-lethal

To spend a long time locked up in a cabin with a face mask will increase the risk for bacterial pneumonia and increase the risk for severe illness and death.