First, it was a matter of he thinks he was taxed correctly in 2018, but it's only recently that Revenue Canada says he was under taxed.
The dispute stems from different interpretations of the US/Canada tax agreement. I don't recall the exact numbers, but he claimed the bonus was received in the US, so he should pay the US rate (47%?), consisting of 15% to Canada and the rest to the US. Revenue Canada says he should have paid the Canadian rate (53%?) with 15% to the US and the rest to Canada. (Presumably if the two governments can agree to the latter version, he can claim a chunk back from the US.)
But I'm not sure why the tax battle will force him to leave Toronto - is the implication that he will run away to the US to try to avoid the issue?