Many theories:

• Trudeau has criticized Trump in the past so it’s just a revenge quest
• Trump wants to be emperor and trying to rule over neighbors is just what emperors do
• Just some delusionial Trump bullshit that demonstrates his mental decline
• (highly speculative) Influence from Elon, whose grandfather wanted to turn Canada into an NRx-style white supremacist corprate-fascist state 80 years ago and failed, hence the move to south africa. Conceivably Elon wants Trump to make him governor of Canada so he can finish the job. Generally I’m not in favor of wild intruguing speculation like this but it’s just too fun/horrifying to pass up.